The Apostrophe Conspiracy

I do not want to be a wanker about this, but the berks of the world have been totally pissing me off (1).

Why don't people know how to use apostrophes? I am in agony reading poorly composed email messages, internet copy and even newspaper articles. Usually, to denote plural, there is no apostrophe. To denote possessive, an apostrophe is placed before the "s" with the notable exception of the word "it". Finally, for plural possessive, the apostrophe is placed after the "s". Examples are shown below:

The cat's whiskers. (one cat and its whiskers)
The cats' whiskers. (a group of cats and their whiskers)

See? Simple, right? Tell me why people with university degrees from English speaking countries can't use apostrophes? In case there is any confusion about my post, I recommend the popular book Eats, Shoots & Leaves by Lynne Truss. I'll let her words echo my feelings on the comma, which are too long and passionate to discuss today:

"When it comes to improving the clarity of a sentence, you can nearly always argue that one should go in; you can nearly always argue that one should come out."

Lynne Truss

(1) http://www.guardian.co.uk/styleguide/page/0,,184840,00.html

berks and wankers
Kingsley Amis identified two principal groups in the debate over use of language: "Berks are careless, coarse, crass, gross and of what anybody would agree is a lower social class than one's own; wankers are prissy, fussy, priggish, prim and of what they would probably misrepresent as a higher social class than one's own"