They might actually expose themselves to more legal liability by banning him (and others) and otherwise censoring posts because, once they start down that road, they become a publisher (content curator) rather than an open platform.

While Trump may not be able to incite violence on the platform if he's banned, somebody else could. The legal argument might be, "Well, Facebook, you took it upon yourselves to make decisions about approved and unapproved content on your site by banning Trump, and then you let Person X post Y, which led to Z. Thus, you are responsible for Z because, based on your own standards, you approved Person X and content Y."

Long story short, Facebook/Twitter, etc., may find it challenging to claim the right to approve/disapprove of content on the one hand, but then also claim they're not responsible for content on the other hand.