The line between acceptable and unacceptable content is not clearly drawn, there is no trial or court system, and if a government was doing the same thing there would be strong freedom of speech concerns. If this continues, more and more controversial communities are going to be shut out of greater and greater portions of the web.
Communities like these are often important for challenging the status quo. It's not too hard to imagine that 50 years ago communities supporting homosexuality would be straddling some line of controversy. For example, Grindr would have almost certainly been illegal.
There are many reasons why it is important to protect the speech rights of people you disagree with. Important progressive ideas very often come from outside the realm of what is considered acceptable to the rest of society. The more we stamp out controversial and distasteful communities, the more at risk we become of supporting and perpetuating regressive ideas without even realizing it. People 50 years ago opposed homosexuality, and 350 years ago any religion outside of Catholicism was not tolerated. We don't have a good way today of knowing what things we do today that will seem horribly regressive 100 years from now, and we won't have a way of figuring it out if we don't allow controversial ideas to exist in our primary thought spaces.