I think it's less about revenue and more about common carrier status. Though FB has recently put out confusing signals [1], it seems that if they start to editorialize what content is delivered, even just a fraction, they are liable to editorialize it all - at HUGE cost.
They currently do all sorts of blocking on what content is delivered. Try and run an ad with nudity. The sorts of nudity that is routine in a European advertising context is blocked on Facebook, because they choose not to. They could easily do the same for political ads, but they don't because they choose not to.
1 https://www.theguardian.com/technology/2018/jul/02/facebook-...