I think this rule goes way way back to when champ spawns were first introduced. I don't know if you guys recall, but I remember masses of people (50, 75, 100+?) at spawns in Destard, Deceit and elsewhere. I remember, when Rikktor was about to pop, dozens of people would be hiding/invis'd around the altar, to get in some damage when he came up. I also seem to recall that, back then, if you died in the spawn area you couldn't re-enter the dungeon for some period of time (or maybe until the boss died).
So, as you can imagine, with 6 power scrolls dropping among 50+ people the odds weren't good; and there were people running around offering justice over and over again to every single player there. It was annoying as hell, not surprisingly quite a lot of player complaints, and I think it was put in to limit this practice.