Since there are legal speed limits in the USA, why is the top speed for cars manufactured for US sales usually 60-70 miles over the speed limits?
Because we have these things called racetracks where people are legally allowed to speed. In fact, they even give out prizes to the winners. 
Cars do have to go up hills. Merge onto the freeway in a timely fashion. These things take more power than cruising on flat level highway at 75 MPH.
so that people will buy them?
The Germans are the market leaders in fast cars. German cars are fast because higher speeds are permitted in Germany.
Higher speeds and the roads for them exist in Germany as a form of subsidy for the motor industry. It is a great deal more expensive to engineer a car for German speed limits than for those applying in the US.
Highly engineered German cars generally (pace Lexus) lead in the prestige stakes and other manufacturers follow. Prestige is the key, imho.
For the run-of-the-mill cars, it’s marketing fluff.
Mom looks at the minivan and thinks how it’s great that she can get all the kids, the dog and a week’s groceries packed inside, and Dad looks at the speedo and thinks “This van will make me look like a dork, but it can go 120 miles an hour! Wheee!”
One reason is the nature of internal combustion engines.
If the maximum possible speed of the car was near the legal speed limit any time you were driving at or near the limit you’d be operating your engine outside the efficient range of the power band.
Think about it, do you really want to have to redline your engine all the time?
Not to mention all of the vehicles that are used by cops, ambulances, etc. that need, on occasion, to exceed the speed limits.
There are long, flat stretches of road out in the less populated areas of the western U.S. where, I believe, speed limits are either very high or nonexistent. (Can any Western Dopers confirm?)
As gazpacho noted, a car that has enough power to drive at the speed limit while driving uphill, hauling cargo, towing a trailer, etc. is going to have enough power to go way above the speed limit under more optimal conditions.
There may be situations (such as when passing on a one-lane-each-way highway) where you might really want the capabilities of putting on a burst of speed, even if you normally drive at or below the speed limit.
The speed limit in Montana used to be “reasonable and prudent” during daylight hours. They changed to 75 MPH in May, 1999.
There are long, straight, flat stretches of I5 in California that have a 75 MPH limit. This is a rare exception to the 65 MPH limit in CA.
Thanks dopers - you guys are good to guests!
For a period of a few years, the law in Montana was just “a safe and reasonable speed”, with no number explicitly mentioned. The Montana legislature shortly decided that that wasn’t such a great idea, and the speed limit on motorways is now 75 mph.
Think of it this way - top speed is determined when engine power matches the dissipation due to aerodynamic forces and rolling resistance; in other words, there is no force left over to accelerate the vehicle (we’re not talking about cars that are gearing-limited in top speed). Typically, those last few mph before terminal velocity are achieved slowly (since you are asymptotic at your top speed). Who would want to creep up on cruising speed, and then be just barely able to maintain it on level ground without a headwind? It would be like driving a moped, or an early VW bug, or something like that - a permanent resident of the slow lane; and pray that nobody ever raises the speed limit.
Basically, look at the design problem from the perspective of what is acceptable acceleration performance for that class of vehicle - that will give you a power-to-weight ratio. Now, frontal area and aerodynamics will come into play to determine top speed.
You are right, top speed is academic for most cars since they will likely never reach it; but nearly all cars will at some point be called upon to deliver peak power (passing, climbing a grade, accelerating with gusto, etc.). Top speed is thus an artifact of power-to-weight (for most cars), and not a design goal in and of itself.
There are devices such as govenors and rev limiters that accomplish the goal of only allowing a car to travel at the speed limit without reengineering everything but I doubt if anyone thinks it’s a good idea.
I look at the way Bill Cosby did, in his album, “200 mph.”
That was a lonnnnng time ago, so I’m just paraphrasing here:
“I live 5 miles from work. I NEED a car that will go 186 miles per hour!”
The exception to the analysis of speed limits vs. top speed are high-performance cars, where the delivered horsepower bears no relation to how fast one can legally drive on US public roads. But that’s, I think, a discussion for another thread.
But, it just falls out that a 4-cylinder economy car is going to have enough power to hit 110-120 mph, easy. Of course, that’s given today’s perceived power requirements, where you can get an econo sedan with upwards of 170 hp and better than 35 mpg. We had an 85 Civic with 76 horsepower, but it would still hit high-90’s in mph. (That was once it was properly broken in, with >200K miles on the odometer. 
)
Just out of interest, wouldn’t it be possible to put some sort of speed-governed cutout switch on an engine that would prevent the car from going over, say, 80 miles an hour, but would also allow a car to operate within the optimum range of engine efficiency below that speed? That is, it would perhaps cut the engine, or reduce its fuel uptake, once the maximum speed was reached.
I’m not advocating such a measure, but my very limited knowledge of our technological capabilities suggests that it wouldn’t be a difficult thing to do.
It exists. My friend had a late 90s Ford Thunderbird that, we discovered on a trip to Vegas, has a governor that cuts acceleration somewhere between 85 and 90.
They sure can, lots of cars (and some motorcycles, for that matter) are already electronically limited to some speed. My Audi hits it’s speed limiter at 130 mph. (Not that I would know that from experience, of course… 
)
I know I’m now treading dangerously close to GD territory here, but I suspect that the reason cars aren’t limited to 65 is for fundraising purposes. Think about it, all those emission controls weren’t put there because the manufacturer or market demands wanted it, they were required by the government. Lawmakers can just as easily require governors, but that would severely hamper a major revenue stream in the form of speeding tickets.
Close, but not quite. The maximum Maximum Speed used in California is 70 MPH. You will indeed see long stretches of Interstate 5 (the Golden State Freeway [TM]) with this Maximum Speed posted on it, but as you say, the Maximum Speed in California is only 65 MPH practically everywhere else.
Note that I say Maximum Speed, not Speed Limit. California makes a legal distinction between the two. “Maximum Speed” is an actual number of miles per hour mandated by law. “Speed Limit” is what it says on road signs as evidence that the maximum safe speed for that particular stretch of road is the posted number of MPH under normal driving conditions. It is possible to contest, in traffic court, that the 30 MPH Speed Limit posted on the sign was too low for that particular stretch of road, and that your 33 MPH speed was in fact below the maximum safe speed. It is not possible in court, however, to contest that the 65 MPH Maximum Speed posted on the sign was too low.