Why do supports get a free pass?

It has become increasingly obvious that supports are becoming what everyone believes that ADC to be, whiny and not that good at the game. It seems that whenever any of the other lanes get caught out, or make a bad move, they get ridiculed for it. But when a support does it, everyone seems to shut up about it. Why is that? Why is it that supports get a free pass to be bad, when the other lanes make one or two mistakes and get yelled at? In game I can see this trend becoming common, but the boards are where it thrives. People on the boards constantly talk on and on about how supports are unappreciated in the community and that the ADC's are spoiled little children who have bad tempers. But this does not seem to be the case for me. In most cases, the support is nastier than the ADC. So, why is this exactly? Why is it that supports get a free pass on bad behavior and being bad at the game? If I try just as hard as the support to win, but I'm NOT a support, why don't I get a free pass on mistakes? I need somebody else to explain this to me please/