I wear the PT shirts because honestly, they're comfortable. I have an AF one and a Marines one out of respect for my father (who was AF) and a good friend (marine.)
I also think the BDU pants are comfortable too, so I'll wear those out and about.
I don't think the shirts are disrespectful, in fact, I think it can be done as a sign of respect to those who've served by showing support.
I agree pretending you served when you didn't is dumb.
C'mon. Didn't or doesn't anyone play army anymore? I remember I did it as well as all the other kids on the block. You know, dress-up in your dad's or uncle's old army uniforms, grab your coolest plastic machinegun, gear, etc. and go out and kill all the enemy that's invading and hiding around the neighborhood.