Actually, everyone does say "I could care less," instead of "I couldn't..." It really is ridiculous. But unfortunately, we don't have you to correct the whole fucking world of their ignorance, Leandro. =)
Well then "everyone" you know is fucking wrong. As the only way that I have ever heard the phrase used is with 'couldn't'. It's not even just incorrect grammar, the phrase "I could care less" implies the exact opposite of the context it should be used in.
In other words, don't get all fucking sarcastic when you've got nothing to be sarcastic about.