Risking your job when you live hand to mouth and jobs are scarce is brave. Risking your job when you're a multi-millionaire on a multi-year contract - not so much. Plus there's this aggressive culture of machismo among Black males, particularly athletes. They don't care about threats. And as pampered pets since they started showing promise on the football field, I don't think they're all that worried about real repercussions.
I agree that there is less risk, but also it's different: any time, even one game off, during your prime years reduces your potential for fame, achievement and money. Also, I have yet to see a correlation between NFL earnings and willingness to kneel. In fact, I'd bet that high-visibility QBs are less willing to make waves, especially white ones, than linemen.
Your idea that they don't 'care about threats' really says they're not too bright. But you're opening up a door to discounting bravery in anyone based on group demographics really. The implication is that they're not to be admired as much, correct ? IF you follow that idea, though, it takes you to weird places. Take race out of it and you can say something like "soldiers aren't really brave, they're just dumb young guys".