One of the first things they teach you in driver's training is how to park. Now if you mess up parallel parking, that's one thing, as you have to be some kind of wizard to get that right on the first try. But when it comes to simply pulling into a spot clearly marked with lines on either side, it's not exactly like landing the space shuttle.
Mistakes happen, but sometimes you can tell people are just idiots and couldn't care less about how their car is taking up two spots or a reserved space. That's why there's vigilante justice in the form of strongly-worded notes or actions, as these people clearly demonstrate.