Where does the constitution say anything about free speech?  That's in the bill of rights(1st amendment and all) and doesn't say a thing about it not applying at work.  

Obviously an owner can fire an at will employee for any reason they want(barring discrimination of protected classes).  If you want them fired, complain to the owners.

Why do you even care though?  For all the right bitches about political correctness and how liberals are too sensitive, you seem awfully butthurt because some people didn't stand during a song you yourself don't stand for.(Don't tell me you've really stood silently during the anthem before every NFL game you've ever watched, you'll sound silly)