Those are private companies, they can choose who or what they post. They are not infringing on anyone's rights, those people are still allowed to express themselves as they wish, only not on that platform.
Being businesses, they want to sell their product to a wide a base as possible. If an intimidating atmosphere or one where members are being outright harrassed occurs, it is in their best interest to intecede. Public pressure as well as from sponsors is another factor.
The owner of Reddit himself finally resorted to trolling the site to expose and remove trolls who were ruining it, to the point that some had been trolling him and even threatening his life.
Such an ugly climate has infected this site (and yes, some may argue that I'm part of that). That's what happens with social media, a site grows, becomes popular, and ultimately trolls take over chasing others away, ending in the collapse of the site.