Just because you take it as a "low blow" and feel butthurt over it doesn't mean that's the intention. If you see someone doing something you know is wrong, you'll at least acknowledge it to yourself, but how is said person to know it's wrong unless they're told that it's wrong?
There's a difference between typos and people getting words wrong. I think there isn't much in the way of excuses for saying things like "brought" when they mean "bought", or "defiantly" when they mean "definitely". They are different words with completely different meanings.
Those that go on about how it's not important to spell correctly "on the internet" are just kidding themselves on how unimportant it is to use their own language properly. I know full well that these people would spell exactly the same if they were applying for a job, for example.
English isn't hard, especially not for those who are native speakers and people who go on about being dyslexic need to stop as well, it's said so often that I ignore most claims of having it as an excuse for poor English.
Just think for a moment how much it annoys you when people correct others, and consider that the "Grammar Nazis" feel the same annoyance when someone clearly can't be bothered putting the tiny bit of effort it requires to use proper English. It's really not hard if you stop being lazy.
Also, stop assuming people will only correct someone as a way of "not losing a debate", that's just an example of someone trying to divert attention elsewhere. I can back up my own debates and arguments whether or not I've corrected someone, and I'm not going to correct someone to divert attention elsewhere, you're just looking for ways you can further complain and belittle those who correct others in pretty much the same way you're complaining about.