Why do people criticize other people's grammar? What do you think of them?
Is that not why you are on italki?
Em... Do you mean here or in real life? Because if you mean here, then people are just trying to be helpful, but if you mean the latter... I don't know. It's hard to say why. It's just that you know how to say or write something 'correctly' and it irritates you when other people don't. I mean, if you honestly think that it's a shame not to know something, then sometimes it's difficult not to criticise those who don't even make an effort to overcome their illiteracy.
But I must say that my brilliant career as a grammar nаzi which I started in primary school when my classmate gave me a birthday card saying, "С Днем рождение Ксенья!" instead of "С днем рождения, Ксения!" (I can't believe I was so cruel to point it out to him...) has derailed since I started learning other languages. Because yes, guess who's writing illiterate nonsense now...
Students PAY ME to criticise their grammar.
Perhaps your question relates not to italki but to some other situation? I mean, if someone shouted, "you've just split an infinitive!" when I was in the pub having a spot of grub, I'd be a tad huffy.
It's rude to do it in non-language-teaching environments, unless something is impossible to understand.
In language-teaching environments, it should be done without making someone feel bad or unexpectedly. You can't learn everything at the same time and so it's not always helpful to correct grammar when the student's focus is on a different issue.
Correcting grammar is only one part of teaching, and correct grammar is only one part of effective communication.
