If it seems arbitrary that's because it is. He knows a little about vampires and angels thanks to Joker, but nothing in huge detail. He knows that becoming a vampire changes you but stuff it. As far as 9S is concerned, they're still who they were before. Just like when he was a chicken, he was still an android at (metaphorical) heart.

"Yeah, yeah," he says dismissively. Whatever you say, Cole. It IS that simple. But seeing an opportunity to get out of this conversation, he points out the kids in the park, playing happily in the water. "Anyway, after the whole thing with the cat and mouse, I thought you'd like this place better."

The fact that 9S enjoys seeing the happiness of humans is just a side benefit.