You raise an interesting point NY... I personally have no problem with what he stood up for. How he went about it is really the issue. In a place of employment, there’s really no such thing as freedom of expression, freedom of speech, etc. Companies have Code of Conduct and other policies for a reason. Whether it’s the NFL or Walmart, you can’t dress however you want and hide behind “freedom of expression” for example. Nor use the workplace as a political platform. Employers have every right to fire people who do (workers are protected by EEOC rights, not constitutional rights).
Kap might've been more effective if he organized public rallies or maybe gone on the cable news or talk show circuit (celebs do it all the time). The mistake he made was “bringing it into the workplace”, which is a no no. Being a celebrity athlete may be different than a Walmart door greeter, but not when it comes to employment law. The NFL is just another U.S. corporation, and Kap was an employee. If he violated a corporate policy, they don’t have to hire him if they don’t want to. This isn't a polemic, it's just the way it is (I'm only calling balls and strikes – don't kill the messenger).
(Sure, he also angered the right wing by “disrespecting the flag”, but that’s a separate thing).
That said, I’ve never heard anything about Kap being a problem in the locker room. Has that been reported?