A slave contract wasn't a simple sheet of paper. The terms and conditions listed on it would decide the future of the ones signing it. But it was more like some sort of trade, in which the humans willingly gave away their loyalty in hopes of getting food and shelter.
Strangely enough, the invention of the contract wasn't done by the werewolves or any other factions, but by humans themselves. Why would they do it, you ask? Well, simply because in the past, their treatment was much worse than they usually suffered through nowadays.
The evolved beings were able to use their strength to dominate them and make the lives of humans a living hell. Thus, the humans willing came to an agreement with the werewolves, they will willingly give away their loyalty to them, in exchange for certain things like food.
