Memes and attitudes are often forced on us, by parents, friends or the things we read. Once we get into a certain point of view, we like and remember things that agree with that point of view - the memes* become self-reinforcing. (Humans like identifying patterns. I'd even go so far as to say it's the main component of intelligence.)
So our point of view often depends on which part of society we come from. Who, then, makes up points of view in the first place? Leaders. Leaders are often strongly biased one way or another by existing points of view and can make (very) bad judgements, or judgements that are good for them in the short term but ultimately very bad for {[many] other people|those around them|themselves|society}. **
All this leads to ideas that are more trouble than they're worth becoming widespread.
The best defence is critical thinking. Analyse ideas yourself, carefully. Check them as much as you can. There doesn't seem to be enough of that going around these days. Internet culture (tl;dr) doesn't encourage it.
Censorship is the antithesis of encouraging critical thinking, and is *very bad*.
* I apologize for using this word repeatedly, but I couldn't think of a better way of putting it.
** The reason for the bad decisions is often extending a previously recognised pattern into a domain where it doesn't apply.
