Because a bunch of kids wearing American flag colors on that particular day would be like them saying "Screw your Cinco de Mayo, this is America!"
No one cares what you interpreted it as. I can interpret the principal forcing those kids to take off the shirts as saying "Screw America, this is Mexico's day to celebrate!" Why is my interpretation any less valid than yours?
The fact is that we can interpret things any way we want. We shouldn't be telling kids to take off a shirt just because it depicts our nation flag on a certain day.