Probably the same reason why someone’s pride in homosexuality would require an open proclamation.
Why does something need to be “required” in order for it to be “allowed”?
If someone’s proud to be straight, they have just as much right to proclaim it as someone who’s proud to be gay.
(Note: I am not trying to excuse the kid’s behavior in the OP… I think he acted like a jackass upon being confronted about his shirt, and he duly got punished for it)