I'm sure twitter is doing extremely well with Trump utilizing them. Their only concern should be if their service becomes deemed a piece of national infrastructure because the President uses it for "official business". That might effect their overhead in some way due to regulation... or even effect their ability to maintain a revenue stream... but that is very unlikely. Also, there is no chance they would ban him, no matter what. They have nothing to gain and much to lose in doing that.
At the most, they might come under scrutiny for not banning him for things others have been banned for... but they would shake that right off, no doubt.