Actually most of the people were presuming MZ is evil and incapable of doing anything without personal gain, rather than most rich people.
Yes, that's unfair. However, since MZ controls FB with his >50% holding, he is personally responsible for the continual bait-and-switch privacy behaviors at FB which no-one can claim is nice. Note also that most of this pattern occurred before FB had a 'fiscal duty to its shareholders'.
So it's not unreasonable to ask for a higher level of evidence before believing that BG or MZ are behaving altruistically.