Some people, clearly! A day doesn't go past where someone doesn't correct one of my typos, smugly pointing out my flaws in the English language. Sometimes they're just being helpful and, indeed, it is necessary but a lot of the time it is completely unneeded and plain annoying. Certain occasions where spelling does matter and correcting a typo is obviously important - on a blog, for example, where the whole point of engaging with that website is to read something. It would help if it was typed correctly - which is why I always apologise for typos in my blog and correct any that are pointed out to me (and I do try and proof read, but I am hopeless at it!).
Maybe I should lighten up and ignore it, but I really don't think that is the issue. It is much easier to look past a typo, ignore the mistake and type out a reply. Maybe it shows a lack of conversational skills, or a desire to be "humorous" but correcting a typo is not the way forward in a conversation online! Assuming you knew what I meant, is there a problem?
"BUT YOU NEED TO LEARN!" I hear you cry as I accidentally type form instead of from or too instead of to. And sometimes you'd be correct. It took me 7 goes to spell "accidentally" there, convinced as I am that it is 'accidently' or 'accidentely' but that isn't a typo. That is me not being able to spell and I am the first to admit in a message when I can't spell a word. And there is a difference between typing "I just come form home" and typing "I love you're mum" because the latter does imply misunderstanding that can be corrected. And even then, you can type "you're" when you mean 'your' just because your brain is working faster than your fingers as you type and don't realise you typed 'you're' by mistake.
A typo is a genuine mistake, usually derived from typing to fast or not quite being able to touch type properly. I could not care less if I accidentally typed "get's" when it is "gets" and I know I did not cause you major offence. You understood what I meant. Life goes on. Conversation wasn't halted. Stop correcting me. Rant over.