It becomes censorship when a moderator removes a comment or discussion based purely on personal beliefs and disagreements.
Moderation should be about removing the trolls, not what it has become, which is censorship.
It feels like many people personally enjoy moderating down disagreements because they don't get any actual power in their own lives.
It's one of the reasons I stay away from most online discussions these days: because nobody can be open and honest. It really makes me wonder if this is the reason why secret groups like the masons were created. Back in those days, instead of getting moderating down, you were killed or attacked.