If you like Mexico so much that an American wearing and American flag T-shirt offends you maybe you should go back to Mexico. I don't understand why in the USA we place and importance on ANY other countries holidays except for noting them on the calander.
I always thought Cinco de Mayo was just an excuse to get bombed on $2 margaritas and pull out that studip, over-sized sombrero you got on a spring break trip to Cancun back in college. Anything more than that I don't give a sh^t b/c I live in the USA, not Mexico.
As for wearing the flag, from the way I read the posted rules then NO ONE should wear it. That would includes the military and all the cops and everyone else who tacks one on to whatever uniform they wear. Not only that, but 95% of them wear it wrong to include the Boy Scouts.