Why, when people attempt to insult intelligence, do they almost ALWAYS make a grammatical.....?

Because not only the one they are insulting, but also they, are products of the American Education system, and their part of the system may suck so much that they actually think they've spelled the word correctly or used the correct grammar. On a similar note but related to math, I saw someone who was taught greater than/less than symbols incorrectly in math when I was in elementary school.