The short answer is probably that if Twitter were to ban Trump, they'd face an enormous backlash from folks who take it as suppression of political opinions. It's a blessing and a curse for twitter, I'm sure -- having Trump tweet regularly has probably been a boon for business, overall. If you're generous, you could make the argument that Twitter is allowing the President to bypass the traditional media outlets and have his voice heard directly (for better or worse).

> Surely threatening nuclear war violates their "Abusive Behavior" rules.

Even if you were to try to live by the letter of the law, you'd have a hard time really getting this to stick, I imagine. If this is the tweet you're referring to:

"Just heard Foreign Minister of North Korea speak at U.N. If he echoes thoughts of Little Rocket Man, they won't be around much longer!"

Then I think you'd have a hard time construing this as "terrorism", any moreso than half of Twitter's population has said (sports teams getting "killed", public figures "not being around anymore" (i.e. probably meaning "in office")).