Why do people hate alignment?

I've been thinking about this recently. Why do people hate the alignment system so much? I hear so many people ragging on it: "it limits roleplaying!" "it confines your character!" "it's unrealistic!" I don't get it. Alignment isn't supposed to be a restrictive thing. In a world of good vs. evil with a black and white ethics system (like most D&D worlds) alignments make perfect sense. They only limit roleplaying when you start acting like a paragon of your alignment. An alignment should represent the actions of your character, not the other way around. That's why DMs have the power to change alignment in most games: the player creates a concept and tries to find an alignment to fit it, and then the DM changes it if they think the person choose incorrectly. I say the biggest problem with alignment is that people use it wrong.

The funny part is, though, every game has alignment! Everyone recognizes the need to catagorize and score a persons behavior, but now instead of restricted "alignments" they have sliding scales that show how corrupt you are. So tell me, why is it so much better for roleplaying to say "I have a humanity of 2" instead of saying "I'm chaotic evil"?