This doesn't really surprise me. The Weeknd got his sound from his original producer and then kicked him to the curb. Drake helped him get noticed and he said "whatever" and signed with someone else. I like his music, but he obviously has no problems with using and abusing other artists to get ahead.
If you've actually heard the original 3 songs that his first producer made, which by the way are probably the least "dark" of anything else on HOB, then you would know that his first producer didn't "create" The Weeknd's sound. Not to mention he didn't ditch Drake, because Drake said himself he knew full and well that The Weeknd wouldn't sign to OVO. Besides that even, OVO is signed to Universal Republic which is what The Weeknd signed to, so they're basically on the same label anyway, just The Weeknd's money isn't going through 3 labels before it gets to him.