It's more the psychological aspect of knowing that something is always watching and knowing where you are. There's a creepy-feeling mental overhead to it all.
Exactly, remember at school when the teacher stood behind your desk and looked over your shoulder whist you were writing something, doing sums etc. The teacher standing there watching evey move you made was a surefire way of you making a mistake.
Even my partner's cat will stop eating its meal if I stare at it whilst it's eating (yes, it's a bit neurotic). (But unlike Google, Facebook, the Government etc. I'm immediately in trouble for teasing the cat.)
