To make that more clear,
I think freedom of speech means no one has to stand or sing the national anthem, there is no enforceable law on the matter. However there is neither is there a law protecting that person from consequences of doing so, such as getting fired or not getting hired.
Personally I don't think that's a good way to protest, I think disrespecting (or perceived disrespect of the) flag detracts from whatever cause you are trying to promote. I don't at all feel bad for Kaep or any other who suffers the consequences of their actions.
In regards to Jefferson, I think he's a terrible football player (I've felt that long before this thread) and this won't matter much going forward.