I normally don't argue for censorship but I will throw out that if you believe the following things:
1. There exist thoughts and beliefs which are dangerous to (my) society
2. These thoughts and beliefs are normally tempered through social pressure in the dominant culture
3. Through the internet you can create your own subculture which reinforces your thoughts and beliefs to become the new normal
Then it makes sense to try to stamp out any places where you can form these groups, since you (the dominant culture) want to maintain the status quo.
This isn't necessarily unique to the present day politics, there was a time when civil/economic rights were a radical faction in a broader culture which would want it stamped out too.