Rule # 1. Rights have responsibilities and consequences.
Rule # 2. The Bill of Rights set forth rights of citizens that pertain to the government.
People do not have free speech rights during periods of work, whether for private entities or the governments (see, e.g., Hatch Act).
As to sports, if the owner/coach (pro ball) or coach (college ball) says no kneeling, there may be consequences if the the player kneels. The player can decide what they want to do but has no ground to bitch about what consequences come his way. They have no right to free speech other than what may be in the collective bargaining.
Since this thread is about fan reactions, whatever the fan reaction is, it is legitimate. Support it, don't support it, cheer the player, boo the player, buy his jersey, burn her jersey, don't be offended or be offended by it. It's all legit.
Query, if you are offended, does it mean that in today's woke climate you should feel free to tear down any team statutes or deface the player's picture on team posters spread around town; or burn their vehicles; or storm their apartments? Isn't violent protesting of the peaceful protest the American way?