I kind of take exception to the idea that "act like grown-ups" means "prefer engaging in ritual displays of conformance to getting real work done". I mean, I associate the former more with acting like exceptionally image-conscious high-school students rather than acting like grownups.
That's not to say that dress rules can't have something to do with acting like grownups; even if it was miguided, the one place I worked that prohibited technical staff from wearing ties on the basis of published research showing that wearing ties reduced the output of technical workers had a grown-up reason.
Here's an example. The top hat (http://en.wikipedia.org/wiki/Top_hat) was a standard part of formal dress in the latter part of the 19th century, but it had mostly fallen out of everyday use by the mid-1910s. Diplomats and statesmen kept on wearing them at official occasions, though, all the way into the 1950s (see http://en.wikipedia.org/wiki/Top_hat#20th_century).
Why? Because they had become part of the definition of "what you wear to an occasion of state." That type of distinction, once reached, takes a long time to fall away; no diplomat wants to be the first person to cause offense by ditching the top hat. So it had to be dead in the everyday culture for decades before leaders felt safe enough to stop wearing it.
Suits are probably on a similar trajectory: once an item of everyday wear; now mostly a way to signal that you're on your way to a Serious Event (like a wedding or funeral or meeting with a head of state); eventually just something people wear to Serious Events only because that's what you wear to Serious Events; then finally (long after they became obsolete everywhere else) left behind even for that.
You know what grown-ups see as demonstrating respect for an institution? Devoting a substantial portion of your waking hours to performing quality work on behalf of the institution.
If you prefer ritual displays of conformance as a mark of respect, that's a choice with consequences, and either not recognizing or preferring those consequences is not a sign of being "grown-up".
I say this as a non-conformist in many areas. But I conform in many that bring me no pleasure. For instance, I don't meet clients in shorts or pajamas, though either would be more comfortable.
What's the difference between that rule and wearing a suit at the White House?
Respect doesn't have to be an exclusive either/or between ritual and quality work.
I was a non-conformist in high school and much of my undergrad, but you'd probably never have known it, as I wore things which were comfortable (and happened to be inoffensive) rather than going out of my way to dress in the same uncomfortable things that all the other "non-conformists" wore to show society they didn't care, man.
True story: I worked for a while in the office of a Midwestern Senator who felt very strongly about this rule. He'd actually fired people in the past over it, apparently. But at one point my boss (his communications director) needed someone to help with a press conference he was holding in the Capitol, and I, who was not wearing a suit, was the only one available. My boss decided I would do because the Senator wasn't going to be at this particular press conference, so he'd never know. So off we go to the Capitol.
Then, halfway through the press conference, the Senator ambles into the room, unannounced and unexpected. My boss' faces turns white as he realizes that if the Senator realizes I'm from his office -- I was too far down the totem pole for him to recognize me immediately, thank God -- we're both screwed. So to protect us both, he does the only thing he can do: he physically grabs me, pushes me into a wiring closet, and closes the door.
This was surprising, but at least I was smart enough to stay in there and keep quiet until he opened the door and let me know the coast was clear...
A big part of this job is interfacing with other government agencies, which means you're going to visit their offices.
FREEMAN: Oh, you wanna know why? I'll tell you why. It's symbolic. The management wants you to know that you're their dog, so you're wearing their leash. You don't see me wearing a tie, do you?
I worked in the headquarters of a bulge bracket investment bank in Midtown and in my experience it is uncommon for people to wear ties even there.
Further, in my estimation, for most groups, it is socially unacceptable to wear a tie every day unless you are a certified BSD executive.
Guess this is what causes culture clash :)
Not to mention most tshirts don't fit female bodies in an ideal way anyway...
Personally I never understood the "dress down" culture in programming. I dress up (sports coats, shirts, nice jeans) because I like it.
Likewise, though I trade out the jeans for slacks. I don't like jeans with a sport coat (on me - you can wear what you want).
And then there are suits and non-suits who care strongly about whatever brand of etiquette they adopted.
Now guess who will also put up a fight over harmless technical/business/political/you-name-it minutiae.
There's lots of different contexts that programmers work in. In some of them, that's true, in others its not. Plenty of places, khakis and sports/polo shirts is a lot more common than jeans and t-shirts. And in others, its suits.