Well, they may not have intervened in previous wars, but on the other hand those wars didn't bring us close to extinction. As was mentioned, fallow slaves can't have weapons of mass destruction. But what about other weapons? I mean, humanity has just been pretty much beaten down, had it's pride and future taken away and it's nose rubbed in the fact that there is no future but this godforsaken ball of dust. Unless the Arilou have changed us a lot over the last 100 years, there's going to be plenty of pissed off people around, people who are ready to take on the central regime that lost the war and the future.
So what if these people don't have real weapons of mass destruction? We are weapons of mass destruction. Sure, the Ur-Quan might come around and reset the balance every 7 years, if they care about the central government of a fallow race. But so what? New scum will always rise to the top. Eventually, we will have wrecked the planet again, only this time there is no place to go. Either the Ur-Quan finally gets annoyed enough to wipe us all out, or we make life on Earth so hard that we just don't make it.
Would the Arilou be satisfied with only the Vega colonists? Are they enough for whatever the plan is? Or would they have to step in on Earth every now and then and tip the balance towards order before we descend into complete chaos?
Or maybe I'm just being too pessimistic.