How do they ban it?
It's a US based company. The free speech provisions of the First Amendment prevent federal, state, and local governments from directly censoring the Internet (except in extreme circumstances, for instance content that directly leads to the harm of children).
Again, people really need to get their head around the simple fact that anything and everything that the current board of twitter has banned, got banned because it was making Twitter legally liable.
The USA has section 230 which protects them from most stuff, but the USA is only 1/5th of twitters audience and not the one it wants to grow.
If it doesn’t moderate itself, it will get banned in the EU, India and Japan.
The key thing about section 230 however is that the more action twitter takes to moderate the more it becomes responsible for the stuff that is left there.
So by making themselves acceptable to the world outside the US, they lose their protections to not act inside the US.