"but it should not be used to insult or offend other people
I appreciate everything you said but I think that this is the time when the First Amendment is most necessary, when what you are saying does not fit in.
A political faction gets into power and it seems that the first thing they want to do is to find some way to shut up the other side.
Standard phony spins include "the other side is being divisive".
On the university campuses they have made up this thing called "principles of community" under which something that goes against their politics is considered in violation.
*
As for the players I think that it is sad they are protesting the system that has allowed them to rise to where they are.
As for sanctions, I think that it is up to their employer, who has the right since they are doing this while on the job.  Tell them to go kneel in the parking lot after the game is over and they are on their own time.