For that matter, how do we know for sure that the whole Twilight thing wasn't their plan from the moment that Whistler first recruited Angel in that alley, or even as far back as when the Gypsies first placed their curse on him? They've been manipulating him for years, tricking him into doing things for them while making him think that he was acting on his own free will. Why not assume, or at least consider the possibility, that the Powers aren't necessarily the good guys they've always pretended to be? It would work in perfectly with them pulling him back out of hell in Season 3, rejecting his humanity in Season 1 of his own show, and even his selling out to Wolfram & Hart (especially if the Senior Partners and the Powers were secretly in cahoots together all of this time.)
(That's part of what I like to refer to as the Powers Conspiricy Theory, which I may elaborate on some day)