Because these extremely small set of activists include the UK’s government and the newspaper The Guardian who pulled their ads due to YouTube placing them next to “extremist content”. Not to mention the numerous US government watchdogs who have been sniffing out a way to thwart google and Facebook for years now.
As far as actual extreme reactionary content.. I don't know what the solution is. I'd put "the middle" as stop promoting them because they hit your engagement metrics, but be much more cautious about banning them, giving them martyrdom points and an incentive to organize better.
Ultimately the villain here is engagement-optimizing recommendation algos. The current situation is a small reactionary channel will be promoted for engagement, until it's big and people notice, and THEN it's banned, with a maximum of negative impact. Worst of all worlds.
There's probably room in the middle somewhere.