Why Is This Simple Grammar Rule SO Difficult?

I've started keeping a collection of writers (blog commentors, etc.) who misuse the word "it's" (contraction for "it is"), instead of its (possessive), here are seven examples from just the last several weeks:

1. Shale gas has it's skeptics: they're hesitant about it's prospects based on economics and environmental factors. I don't work in the shale gas industry and therefore don't feel qualified to forecast it's long-term prospect.

2. If oil was so rare in the US, then why is 95% of it's land and water off limits to drilling?

3. My point is that the OECD index uses stock market levels as an input to derive it's levels.

4. Tithing has lost it's promise

5. I suspect my laptop is having trouble keeping up because of it’s GB’s and MB’s.

6. And while you’re at it, do the same with “Price Theory and It’s Applications”

7. I despise Walmart, not for it's anti-union stance, but for it's overall philosophy and what it's done to it's supply base. 

Maybe I'm just a complete supercilious grammar snob, but why do SO many seemingly intelligent people have SO much trouble with a basic grammar rule that is SO simple?  If you didn't learn about this rule in "grammar school," perhaps a simple five-minute remedial review is in order, you can start here.