Why do people feel the need to do that? Quote and correct grammar? I can understand a "ftfy" joke, like changing what they said completely to make a point/joke. But grammar? What's the deal?
Well, because for some of us, it's like nails on a chalkboard. Not that we're not impervious to our own mistakes, but I know I personally would much rather have someone point out my mistake than to let it go unchecked. But, that's just me. Especially if it's one of the higher frequency errors like you're/your or their/there/they're. I know I personally don't do it to be a ***, but... well, I mean it is going to be in my future. As for Spath, he's gonna be grading papers, too. It'll be the kind of thing that makes my eye twitch. So it's not judgement on the person; instead it's more like finding the error and removing it. Kinda like if someone posted: 2 + 2 = 5. Someone's going to correct it because it'll just grate on their nerves. Anyway, my point is sometimes the errors can be a bit much.
And yes, this isn't a formal paper or anything, but people have different pet peeves. I've learned to control mine, but every once in a while, I'll post a gif of that guy eyetwitching. I suppose it's better than pointing out the mistake.