Plenty of people did not pay attention in English/Language Arts classes. Most people are not thoroughly fluent in the rules of standard written English. Everybody, no matter how knowledgeable or careful they are, will misspell words, use a rule incorrectly, or otherwise have bad grammar.
A lot of people I know laugh at bad grammar. I don't. It's not funny to me. It's commonplace. Typing "rabbi" instead of "rabbit" may make for a humorous image, but the typo on its own is not enough to make me laugh out loud.
Now, I will say that I used to laugh at it. I used to think it was some of the funniest stuff around. But having spent as much time on the internet as I have, not to mention as much time editing other people's work as I have, it just isn't exciting anymore. Either I've moved beyond it or I've gone numb to it. Whichever it may be, I know that I'm not going back, and I'm ok with that. There are funnier things out there than typos.