Legally speaking you are perfectly right - they can do whatever they want. But morally speaking they can't - if they have put themselves in a position where they are so popular and making so much money that it is in the general public's best interest for them to censor certain scam artists, then they should invest into content moderators. I have no idea how many people they would need to reliably sort through all the content and only block the scams. But it is the price they need to pay for deciding to censor a specific topic. Otherwise the price is being paid by all the researchers, entrepreneurs, publishers, and enthusiast, who have nothing to do with the scam artists.

This issue basically boils down to big corporations (Facebook, Google, Twitter, Mailchimp) being too lazy to do the right thing.