It is because we are human too, and do our work here in our free time, and don't want to see public debates on whether people think we are doing the job well or not. It can be disheartening to us, and rumours or hear-say can damage the reputations of otherwise decent people. This rule also applies to Game Masters for the same reasons. We are not professionals and do not make perfect decisions 100% of the time, but this does not mean people should vent anger towards us whenever they become unhappy with something. Instead we encourage anyone with an issue to PM us so we can work things out in private, in a sensible fashion. This also solves the problem quicker and easier.
Moderators will not act without reason or cause; this is a rule we must abide by. Admins, such as Vengeance, will never do it either, even though it is in their rights to do whatever they wish.