... no matter how legitimate the issue you are protesting ...
Killing police officers as BLM promotes is not a “legitimate” issue. It’s domestic terrorism. If a similar violent group targets sports broadcasters, and actually killed sports broadcasters like happened in Dallas, I think Mr. Costas would have a different position.
All of this works until you start to have guys do a kneeling to protest the abortion policy of individual states, or they want to go pro-active on Nazi topics, or they want to kneel before each play to protest NFL officiating.