Maybe it's not a big problem in the UK, but here in Texas, it can get pretty hot, and no one wants to be stuck wearing a sweat-drenched T.
At an outdoor concert I went to, at the end of the concert, the girl next to me, who had been dancing through the whole show, gave me a big good-bye hug. Her shirt was drenched in sweat, and I was like, "ewww".
Shirtless is much better, at least the sweat might evaporate if the humidity is less than 90%. And it should be equal opportunity shirtless.
Well, there should be one rule: If you want to go shirtless, you need to have shaved your chest hair, if any. :P