Fair use is a defence in court against a copyright infringement lawsuit.
But Youtube is not a court of law. Youtube is like private property that content creators and consumers have an implicit invite to use. Youtube has only those restrictions required by things like non-discrimination law, so far as what Youtube permits. If Youtube says it won't host some kind of video, what right do you have to insist that they do? If Youtube says it will take down your channel, what right do you have to insist that they don't?
Thus, fair use is not a defence against copyright claims on Youtube. Youtube needs to do the least necessary in law to avoid getting sued, but it is up to Youtube what policy it adopts with copyright content. Something may be fair use in the sense of a copyright infringement lawsuit, but Youtube is not legally required to permit such use on Youtube.
I am not a lawyer, but this is how I read it.
It's like a pub being able to bar a customer for whatever reason (except possibly racial or sexual discrimination). If you annoy the management of a pub, they don't have to obtain a court order to prevent you returning: they simply withdraw your invitation, and once that is done, you are trespassing and they can treat you as a trespasser.
This is the problem with the increasing creep of private ownership, whether land or utilities or social media. Such things are, subject to an restrictions in law, entirely controlled by company board and policy. There is no democratic representation on the board of a company. They do whatever they think will bring them profit, and avoid whatever they think will cause them losses. And everybody else has virtually no say in the matter. Even worse, large private corporations can afford to make large donations/bribes to politicians to bend the laws in their favour, and the rest of us can't afford to donate a dime.