Legally, that's actually not true. You're allowed to protest on the clock too. Then your boss can fire you, not for protesting, but because you're not doing your job.
It really boils down to that. Just like Colin ended up without a job.
The protest's strength is in numbers. Are NFL owners ready to fire all the players that protest, and the backlash that comes with that? Are players ready to unite and continue protesting? Time will tell.
I don't really care either way, I'm not triggered by the protests.