Why was a black hole used to drain energy from something, if black holes are meant to "Drag things in"?
I'm not sure. One idea is the "pure good world" that the black-hole was placed around... would not ever willingly or under any condition, allow energy from itself to be extracted. It was literally too strong. There was no way for the alien/fairy(monster) worlds to drain energy out of it. They would be defeated everytime.
So... considering that that "pulling back" force was so strong, they decided to INCREASE that "pulling back" force... use the determination of that world against itself.
You could see it as being like that world having already "high gravity", a resistance to things being pulled out. So they gave up on trying to pull against gravity and instead INCREASED the gravity there, and just allowed negative-energy to be sucked into that world instead.