If people enter into such contracts, it's because they prefer to be in them.

First, I challenge that. They may not be fully aware of what they are getting into. Second, the very possibility of such contracts could bend the society into generalizing them. For instance, if voluntary slavery is permitted, it might become the only way, but for an elite few, to get a job.

Why do your preferences override theirs?

Why not? My preferences do take other's preferences into account, to the point I would mostly let them live their lives in peace. But the buck has to stop somewhere. For instance, when 2 persons' preferences conflict (like, they are at war), my preferences says something about acceptable resolutions of the conflict. Having both parties risk their lives is not acceptable. I would strip them of their weapons if I could.