I think the excuse for booing is they equate kneeling with the Black Lives Matter organisation which had a manifesto that espoused Marxist views.
Personally, I think this is just an excuse for racists to be racist whilst pretending they're not racist. The gesture actually came from an American football player rather than the Black Lives Matter organisation. And it was because he didn't want to stand for the American anthem whilst he felt the countries police force were racist.
en.wikipedia.org
It's pretty clear to me the players are protesting racism rather than calling for a communist revolution when they take the knee. Not sure how others have become confused about it.