BLM is an amorphous civil rights group centred around highlighting injustice in how black people are treated in society. The knee taking is to do with this. Sure, some more political groups move in to try and co-opt the movement, but I think we should be able to distinguish between football players taking the knee for highlighting racial injustice and socialist groups trying to use it for their own ends. Football players are hardly signalling an intent to be footsoldiers for the revolution.
