because he has some ridiculous OCD rules. This wasn't a kid trying to be defiant, this was a kid trying to pay tribute to a legitimately worthy cause. I don't know the kid, but maybe, just maybe he decided to wear those cleats because he had a family member like his Mom or Grandmother who had to fight breast cancer.
Coaches also have to be willing to make some concessions and allow the players to do some things to actually foster loyalty to the coach, and that actually can make it easier on the coach when he is trying to enforce rules that actually matter, or when he is trying to get them to play as a team.
I don't know that I would fire this coach over this if I was a school administrator, but I would certainly have a long talk with him.