The fact that you're not forced to be on Twitter means that victims can leave, but it also means that bullies can be banned. Why prefer the first option?
I'm not preferring either option. Of course they can be banned. But they will come back. They always do. Why subject yourself to that continuous cycle of stress?
The real question to ask is: why are some of these political nuts working so hard to push people off of Twitter? The answer, in my opinion, is because it allows people they don't approve of the same public voice as themselves, for the first time ever in many cases.