I "license" the apartment I live in, and the state has elaborate enforcement mechanisms for kicking me out when I breach the terms of the license.

Independent invention is definitely a distinguishing characteristic, and the shakiest part of the rationale for patents. IMHO, independent invention should be a defense to patent infringement. That said, it's not really implicated in the drug industry, where the folks doing the copying don't have the resources to invent and test the drugs independently.