Same rule from the AD at our school, though the coach is good about being as flexible as he can be about ignoring the rule when they're on trails or out of town.
As a father, I agree that the rules should ALWAYS be uniform for boys and girls. If someone is going to tell my daughters they can't train in a sports bra on a 95 degree day, then what is the justification for letting the guys go bare chested? There is none.
I can see both sides of the shirt rule, though I personally think it's stupid. ADs get flack from people about seeing the kids out with no shirts, even though the complaints are ridiculous. As long as the rules are uniform for both sexes, kids will deal with it. If it's girls only, it's bull$hit and they should not accept it.