In what world is a contract enforced against a non-participant?

Show me an actual case where a contract (NOT some other actionable matter) was enforced against a someone who was not a party to the contract either as a direct signatory or an agent or participant in a direct signatory.

In your contrived Jeff example, most of the time Jeff is not part of the contract violation lawsuit.  However, as you describe it, Jeff was inducing people to violate the law, which is itself illegal and could be sued on that basis.  Not for violating the contract, because Jeff didn't if he wasn't a party to it!  This is a similar principle to that which finds a pimp, or a 'Fagin' ala Oliver Twist, is violating the law even if not a party to the actual criminal act.