Yes, theapportioner, those networks could do that. They could also broadcast a monkey flinging poo on their stations all day long, at least until the FCC shut them down. But the reality is, they do accept and have accepted terms, in the case of Mayweather and with most other prominent boxers, that do not contain outs of that fashion. Mayweather has an obligation to fight, Showtime has an obligation to air it and pay him, and very few other obligations are material.
As WBV makes clear, there are no other parties here where protesting them would implicitly be protesting Mayweather's existence, as grotesque and incomprehensible as it may be to you. The only solution, if protest is your goal, is to protest Mayweather himself, not watch his fights, convince other people to not watch his fights, and maybe picket or something.
Those disgusted by him for his past moral conduct have already gotten a fairly big win: tons of coverage in the media going over his domestic violence stuff. He's definitely the heel in this fight, PR-wise. But that's as far as it will go.