I give people the benefit of the doubt if they are considered bad for refusing an autograph or chat when they are out and about and minding their own business. If every time you tried to have a bit of privacy someone came over and hassled you then you'd snap occasionally, even if you had the patience of a saint! I'm more inclined to give credence to stories of what people are like to work with.
I used to be on the fringes of the comedy scene and have seen enough to know that some comedians aren't that comfortable in social situations and use personas for on stage, being fairly withdrawn off it. If someone is inclined to put the person on a pedastal, then it's easy to mistake awkwardness for arogance and a wish for privacy as a deliberate snub.
I definitely agree with this. My brother is a big Robert Plant fan, and has seen him a few times at football matches. However, he has never approached him for an autograph, because he says that RP isn't working when he's there, he's just out enjoying himself the same as everybody else.
However when celebs are at events like premieres or anything to do with promoting something they're doing, then I do think they have a duty to be friendly and attentive to their fans as far as they can be.