They are not going on TV and chatting about socio economics, why brexit is bad and how a tax increase could benefit the NHS. They are kneeling to stop a serious problem which is important in and out of politics and not shouting in your face. Furthermore rashford and others go on twitter every day to be racially abused. Are you saying they can't protest that on a public stage?
It’s deeply unwise for them to do so.
What is the end game? When do they stop the protest? Will it go on for ever? Who, exactly, are they speaking to and what specific action would they like them to take?
There will be a day when professional sports teams stop taking the knee, and racism will still be an issue because, frankly, it’s a problem endemic in one form or another within society. There is no way out for them - they will always be seen as having “given up”. Zaha was more circumspect about this and wisely stopped doing it a while ago because he correctly identified it as mostly performative.
Football has a racism problem but importing a gesture from the USA unrelated to the specific and very different race problem Britain has is bound to attract suspicion.
Millwall’s fans booed the taking of the knee (an American gesture heavily associated with the partly discredited BLM organisation and its foul politics over there) but loudly cheered the team when they held up a big, generic banner about anti-racism might be a clue as to what the issue might be. A lot of people really really don’t mind Rashford campaigning for the government to feed children but do mind the feeling of being given a confusing moral lecture - even if they agree with it - by extraordinary privileged multi-millionaires before every game.
I still think the main reason people like taking the knee, and say they support others taking the knee, is because it’s a zero-cost, already mainstreamed gesture far, far removed from Colin Kaepernick’s personally costly, solo demonstration in the US a few years ago.