I don't know who did it. I know demons need to do it to replenish themselves. I could see it being either way though. I mean think about it: you're tied together for the rest of your immortal lives.

[He frowned a little, because contracts were hard.]

Humans can force demons into a contract to. Though it carries a lot of risk. Really, I think all one needs to realize is that they need to think through legal matters like that carefully.