Twitter is not journalism. And as you yourself have pointed out, Trump is one of the most powerful people on earth—meaning he has at his disposal more platforms than any of us can even dream of. Twitter can no more "police his speech" than I can police all of Facebook.

What Twitter can do is decide they don't want Trump to be an active member of their community. That is not just within their legal rights; it's well within their moral rights too.

Should twitter also ban the accounts of the president of Russia and the president of China, because of their support of policies which violate human rights? (LGBT, Tibet, democracy, the list is long, pick your favorite).

What about the account of Barack Obama, who supported extra-judicial drone killings of US citizens?

https://www.vox.com/policy-and-politics/2016/11/14/13577464/...

You're conflating a lot here. People think Trump should be banned from Twitter because of the inflammatory content of his tweets, not because of objectionable things he does elsewhere. Unless Barack Obama made a bunch of tweets exhorting drone strikes that I'm unaware of, the comparison you're suggesting doesn't go through.

If they're contributing to or creating an atmosphere Twitter doesn't want on their service, then yes, why not?

Twitter isn't news, it's social media. I'm perfectly okay with private organizations policing their own content, especially when hosting that content has costs.