It seems to be widely agreed that actions on units or supports can only be activated once per turn unless otherwise stated (I play that way as well). But, what rule or ruling does that stem from? Or, is that something someone made up and everyone just agrees with it?

Actually, exactly the opposite is correct for everything except the "at beginning of turn" actions.

For example, if you have a Nurgle Sorcerer in play, you can pay the 3 as many times as you like.

But if you have a Rat Ogres in play, you can only use the restoring Skaven action once (and hence Rat Ogres + Clan Rats is not an infinite combo). Go check the rules summary post for the link if you like for that one.

Hmm, interesting. Thanks