I can only assume that griefing has crossed their minds and they have some mechanism of coping with it. I am extremely excited about this game but when I read the bit about events scaling...They're talking about how they've solved the problem of kill-stealing but is PvP interaction enabled in public areas? If ten people are "participating" in an event but two or three of them are attacking the other players, that's a problem. Heck, if ten people are participating and two or three are just hanging around doing nothing to make the event scale harder for the remaining players, that's a problem. I can only hope and trust that this possibility is obvious enough that there was some consideration of it in the development of these interactions.
At Pax East, they commented further on this. Kill-stealing is solved because everyone who participates gets full credit and full reward for doing so. They also said that if 10 people are fighting and 2-3 are standing there, the AI is smart enough not to count them in its scaling calculations (they can even plink the enemy a bit to "act" interested...but still not count because they aren't meeting the level necessary to be considered "active" participants). From what they said at the convention panel, it does truly appear like all of these things have been taken into consideration.