I’m pretty sure the problem was not the USMC, it was the dog and the saying. I haven’t seen the shirt, but the saying is pretty crass. Appropriate for a marine isn’t necessarily appropriate for a 6th-grader.
(OK, now I have - I tried to find it and then saw someone had posted it.)
If the standard is consistently applied (which means most concert t-shirts are out), then I side with the principal.
It used to be in my day that any T shirt was cool in school, but all were decent at that time. Again, the total ban is easier than a case by case evaluation of decency. (In the case of the doggie shirt why not just have the boy turn it inside out?) Creeping zero tolerance.