I don't think anybody should be fired or given an HR warning for reasonably expressing an opinion, even at work.

Except if you take it to an inflammatory level and don't stop. Disrespecting the flag and national anthem is as deliberately, in your face  inflammatory to half the country as wearing a confederate flag t-shirt to work is to the other half. 

But Trump is right about the boycott. Vote with your $$ if you don' like it. Take a knee with your $$. Watch how brave the NFL is when their revenue is cut.

On college campuses, the NFL, Google, government jobs, etc. you can broadcast to your heart's content about the liberal laundry list issues. 

But if you're a conservative and try to just as openly express yourself you will get the PC gestapo's S-storm.