The "it's Marxism and politics has no place in football" argument is a complete red herring. If you oppose all politics in football, and the knee is an example of Marxist politics, then you should also oppose displays of capitalism in football which is the direct opposite of Marxism and just as political.
Transfer fees? Booooo!
Having to pay money for your pre match pint? Booooo! Get out of my game!
Club shop selling stuff? How dare you!
Top giving us free beer and cake is more Marxist and that gets lapped up, rightly so.
Identify and ban the lot of the racist knee-booing pond life for good.