Kaepernick wasn't blacklisted for expressing his first amendment rights, he was let go for being distracted on the job. 

Football players are paid to do their job, which is to play football at the highest level. How do you think Exxon would feel if their CEO took a knee for Black Lives Matter before every board meeting? Or how would you feel if your plumber made a speech about LGBTQ rights before fixing your toilet?

Professionals are hired to be professionals, and focus on the job at hand. Kaepernick clearly so many interests outside of the NFL, that being a pro-level QB was no longer his #1 priority. And you can't pay some one millions of dollars to be unfocused. That's just the way the world works.