Former quarterback Tim Tebow’s rabid, irrational supporters undermined his NFL opportunities in much the same fashion as Mr. Kaepernick’s. In 2011 he started 11 games for the Denver Broncos and led them to a come-from-behind playoff victory over the Pittsburgh Steelers. In celebration of big plays and touchdowns, Mr. Tebow knelt in prayer and became a polarizing religious symbol. He was also a below-average passer. The Broncos, and several other teams, discarded the fervent Christian when it became clear his production didn’t justify the controversy associated with his presence.

Mr. Kaepernick’s kneeling is an even riskier proposition. The social-justice warrior has cultivated media alliances far more aggressively than the pious Mr. Tebow. Mr. Kaepernick is also closely aligned with Black Lives Matter media activists. No NFL owner, executive or coach—regardless of race—wants his football decisions second-guessed in the tendentious way BLM activists Monday-morning-quarterback police officers.