That is exactly what happened. I found it odd that the exit rows were open, reservable (I don’t believe that is a word) and didn’t require an extra fee. I wasn’t going to pass it up though (now I know better…airlines LIE). Of course, as I found out, bait and switch. So for you people posting above that fly 1 million miles per year (paid by your employer) and looking down at us lowly two flights per year and saying “derp…you don’t deserve those seats”…the airlines shouldn’t even offer them then.
- Most people who fly lots of miles don’t fly budget airlines that have stadium seating, or b) we tend to have high frequent flyer points and the airlines don’t screw with us so we don’t know or remember what it’s like to be in the thunderdome at the back of the plane.
Delta is budget?
Someday when you retire you’ll get to spend some time with your family. Enjoy your Marriott Suites and complimentary breakfast until then. 
I’m not saying that you don’t deserve them, but from an airline perspective, you’re not worth any special treatment because you bring a relatively small amount of income to the business. That person flying 75K a year or more is bringing in thousands and thousands of dollars of business. Minimum. They’re going to get rewarded for the amount of business they do with the airline. Just like they do with the hotels they stay in for 3 months out of the year, or the 35 separate car rentals they did. /shrug
With respect, I am somewhat surprised at your claim. If the seat was assigned to you then it would’ve indicated that on both your reservation and printable ticket. It seems likely you selected the seat but the system at some point told you that your seat would be “gate assigned” or “not confirmed”. Which is the reservations systems way of saying “uhh… not unless no one else buys that seat or is upgraded to it”. /shrug
I honestly think you simply misinterpreted what the system was telling you. But the system doesn’t flash big red “NO! YOU must pay more for this or we’re not giving it to you!” Nyah! signs. It’ll let you think you have a premium seat (and exit rows are considered premium because of the extra leg room). However, and this is a point to consider that may apply to your specific case, exit row seats also are supposed to be manned by someone. (And they have to be manned by people capable of fulfilling the requirements of operating the emergency exits). So I guess it is possible the system WOULD let you pick the seat at no extra charge and then bump you for someone who paid/upgraded (or was complimentary upgraded based on status).
It is always possible the system had a hiccup, but it shouldn’t have billed you for a premium upgrade and not given you a premium seat. That’d be a violation of the contract of carriage. You’d be owed at least a refund of the cost difference, and probably a bit more.
So I guess it could happen, I’ve just never seen it, ever, in 15+ years of flying and not quite 1 million miles flown so far. And I’ve seen some interesting stuff. 
Regards,
-Bouncer-
I had never seen it happen either, in (checks Delta.com) 469,868 miles, until it happened to me.
I had a work trip to the Netherlands, and my wife decide to come with me. I booked my reservation through our travel office and my wife’s with Delta.com. I then called and had our PNRs linked and confirmed our seats together (in Economy Comfort) throughout the journey. At the time I booked the flights I was Gold on Delta. Two weeks later I had no status, and three days after that got an email from Delta that my seat had been moved on our DTW-AMS flight.
Thankfully the gate agent was able to fix it, but I never had a confirmed seat taken away in 10 years of flying with Delta with Silver, Gold, or (rarely) Platinum status, but within days of having no status it happened.