Supporting the constitution and supporting a specific situation, where silence is held in ceremony to support the people who died in a treacherous event are two different things. Sure, the constitution does give one the freedom to express opinion, but this doesn't mean you should express it in the 9/11 ceremony; have a bit of respect at least.
Those black players are all multimillionaires, so I see no reason why they protested. They themselves have more privilege than a middle class white man, but I don't see them protesting this racial privilege. It's all about popularity and media attention.