I am surprised they can even do something like this. Seems to be a massive violation of personal freedom.
His job is to play football. Can you wear whatever you want to the office (assuming of course that you work at an office)? They restrict a bit before, during and a bit after the event (including the post-game interviews), but they don't tell him he can't use Beats headphones ever.
That said, I find this sort of funny. As others have pointed out, he put them into a pretty good pickle using pink ones to "promote breast cancer awareness." If the NFL has any brains, they'll donate that fine to breast cancer awareness/research.