That being the real cause of the outrage is a dubious claim at best. I don't believe for a second that if he were kneeling at a different time to protest racial injustice, that people would be any less angry.
This was a moment politicized by Trump to help him with his political narrative. And, for the record, I'm no fan of Nike's- I've been boycotting them for a LONG time and will continue to do so. But in this instance- they did the right thing.
But if you look at this situation- it has at least three basic pieces to it:
1) The protest of systemic racial injustice black people (especially men) face at the hands of law enforcement. (Black men account for about 6% of the population and
17% of the people killed by police.
Black people also make up about 39% of the unarmed police shooting victims)
2) A sitting US President trying to pressure people into attacking a US Citizen for exercising their right to protest
3) A black athlete (and now a sports clothing company) using his position (and rights as an American citizen) to remind people of said racial injustice during the national anthem
And if, out of those three, you're MOST concerned about #3- you need to check your priorities. Because #3 isn't the one that's damaging to society.