Proving that a team or a league colluded to keep a player off the field has been difficult over the years.

“Smoking gun” documents that show owners acted in a coordinated way are rare. Aggrieved players have a hard time proving teams deliberately passed them over in favor of less talented alternatives because teams hire and fire players for subjective reasons all the time, and are well within their rights to do so.

Then there is the case against the N.F.L. brought by Colin Kaepernick, the former San Francisco 49ers quarterback who has accused the league’s owners of snubbing him because of his decision to protest by kneeling during the national anthem.

A case that was originally about Kaepernick’s skills has turned into something much larger: a referendum on his politics, free speech and even his legacy. N.F.L. teams have signed players who have beaten their spouses and run operations that killed dogs for sport, but protesting during the national anthem may prove to be the unforgivable sin, it seems.