Right now there are two criticisms of Twitter and Facebook et al:
1) That they are arbitrating content, like what you mentioned with Trump
2) That they are not arbitrating content enough, giving platform to climate change denialism, conspiracy theories, far right radicalization, and genocide [0].
So sure, they can return to their original stance and censor nobody. I don't think people would be too happy with them when the world gets more genocide-y and violent protests break out fuelled by conspiracy theories regarding the health of the elections.
So to recap: moderating and not moderating content on their platforms are both subject to serious criticisms. Right now, social media companies are trying to strike a balance, but the result is that they are angering both sides. If they choose a side (all content is allowed / we will moderate the content as closely as HN is moderated), they other side will be angrier at them. They were happy not doing moderation when the negative impacts were only affecting non-western countries, but now that it is also affecting western countries, they are more clearly seeing the negative sides and they have moved more to the middle.
[0] https://www.nytimes.com/2018/10/15/technology/myanmar-facebo...