In some cases, people think it's rude to suggest to someone that they're wrong.
Suggesting that someone is wrong is a status move -- the person correcting is expressing that they have higher status than the person being corrected. At least this is how neurotypical people would evaluate the interaction. Their model of Harry tells them that Harry believes to have higher status than everyone else, which means that he should be punished to learn his proper place in the tribe.
While directly highlighting a deficiency and offering replacement is a straightforward way there are ways of addressing them. They often require more work but can have such benefits as not triggering adverse social mechanisms. One such route is to get really enthusiastic on the wrong idea and carry it on to it's more extreme side where it can be apparent how absurd the idea was in the first place. The original proposer gets to do the error correction and withdrawal. Thus one can be an active force for truth without being brutal about it.
I was reading reviews of HPMOR on Goodreads and I noticed that the people who didn't like the book were essentially "put off by the rationality". They thought Harry was arrogant and condescending.
Then I was thinking, a lot of people are "put off by rationality" for similar reasons. What a shame. There's a lot of value in spreading rationality, and this seems to be a big obstacle in doing so.
Any thoughts on how to make people less "put off by rationality"? I think the core issues are: