Anyone who doesn't know what an apostophe is for (or what one is, when you tell them to put one in) clearly skipped a lot of classes and ignore all professionally written media. People who ignore things are, by definition, ignorant.
Well said. I'd also add that such people are often:
1) Lazy, in that they refuse to learn how to correct their simple mistakes.
2) Arrogant/aggressive/defensive, in that they often respond angrily or defensively to people who point out their mistakes in an effort to help them improve. “Grammar Nazi" is a common label for people who prefer clear and effective communication.
3) Selfish, in that they don't care if their text appears confusing, garbled, or incomprehensible - they are happy to let other people work at deciphering it.
4) Vulgar, in that they're also perfectly content to appear illiterate and uneducated.