For these guys, it's because they make wide right-hand turns. Sometimes, the only way for them to make a right turn is to swing to the left first. Stupid people will sometimes try to squeeze by on the right when they do that, even when the driver has his blinker on, and then get squished when the driver tries to make the turn.

As for why you shouldn't pass on the right normally, bear in mind that these rules were written years ago, and that when they were written, many cars did not have passenger side wing mirrors. Even though everything has a passenger side mirror nowadays, they bear the warning, "Objects in mirror are closer than they appear." So if you go to pass someone on the right and that driver decides to move back over, then when he looks in his right side mirror, it looks like he has more space than he actually has.