Nowadays, the hat is likely to be a backwards baseball cap. Not worn by some kid, but by a guy old enough to know better. (Shielding the face from sunlight is actually a good idea.) Maybe he’s trying to hide his bald spot. And taking the hat off will just wreck his combover. (I have nothing against bald spots. Combovers are a different.)
Here in Texas, cowboys with real rural roots usually know proper manners. Fratboys in cowboy hats, not so much…
And hare brained (in another post)
Where, where?
It’s not as simple as you might think:
I always heard that you took off your helmet to show your peaceful intentions as well as your belief that you were safe (i.e. you don’t need your armor)…so, it became a sign of respect. It goes fairly far back, so it’s not exactly some quaint American custom to doff ones hat as a show of respect.
BTW, I think Shodan’s comments aren’t all that far off the wall…I know that wearing a helmet with plumes and such was a way to make one appear taller, so doffing a helmet to show you are shorter seems logical as well. The exact reason is probably lost in time though, since a quick Google search turns up as many answers as to why as there are links (212 million according to the top of the page)…
-XT
Taking my hat off is just easier than getting the shit beat out of me by a large tattooed biker with a shaved head and German helmet.
My head isn’t shaved and I don’t wear a “German” helmet, other than that, a fairly accurate description. 
(And I see that you are also, a wise man!) 
And a prodigious poster, to boot!
I noted my post count with shock the other day. All those words, so little content.
Meh! Don’t be too hard on yourself, it’s the thought that counts!
My only defense is that I’ve been retired for three years. Prior to that, I was one of those useless, lazy government contractors and public servants that we’re reading so much about on the board these days.
Well… I, for one, won’t hold that against you… 
Somebody has to be… uh… well… what you said…
My turn, to ::nitpick::!
IMHO, the “proper” snarkastic response to “hear, hear”, should have been… “What, what?” 
Just sayin’…
I have heard in several places that the military salute comes from days of armored knights who would raise their visor as a gesture of respect. After soldiers stopped wearing visored helmets, they still raised their hand as if they were about to raise a visor.
This could be related.
^
close. the gesture of respect is in showing one’s face.
NB: you are not fully recognizeable when you’re wearing a hat (there are exceptions, the joker for instance.)
This seems the best answer to me.
I remember one Sopranos episode where Tony, uh, suggests that another diner remove his cap.
As long as I’m getting history/culture here, and even if for historical purposes only, check a great and funny/nasty scene where Hamlet teases a courtier mercilessly to put his hat on–as a sign of respect–during a brutally hot day.
My understanding is raising the visor was to aid in identification. It’s very hard to see a knight’s (or cataphract’s) face through a proper helmet visor, hence the development of various methods for armored cavalry (the real kind, not the motorized variant) to figure out whether or not to kill each other.
Because it’s the law. (Although I don’t think there’s a penalty for breaking it.)
So wait, that only applies to men? Women wearing hats need not remove them?
The fact that it appears in the U.S. Code means it can technically be described as a law, but it really has none of the other hallmarks of law. As you said, there’s no way to enforce it. It says “should,” which is at best ambiguous and at worst entirely voids any obligation whatsoever. Most importantly, if it did have any of the compulsion characteristic of a law, it would be struck down as unconstitutional.
I haven’t found any other portion of the U.S. Code to be a pile of bullshit as this one is.