I think one of the reasons may be more patient with those backing out is that it doesn't seem so deliberate. If you chose to back in, when there are people waiting behind you, you are knowingly causing a delay - you could chose to drive in and there might be fewer people around when you leave, so you won't hold anyone up.
If you're backing out, it doesn't seem so much as though you're ignoring the queue, as you don't have any alternative (plus, they may be waiting for the space ;-) )
(all 'yous' general)
My personal preference is to back in rather than out as I think it is often safer, as you have better visibility, but if I arrive at a busy time I will go in forwards (or f the car park is busy but not full, I may go further in - I fnd often the car behind me will rush for the space I *didn't* take, and then it is them holding up the queue, and I get to park a little further on with no-one breathing down my neck!