I think the term "IP" is meant to highlight the one big similarity they share. We recognize that this thing, which is more of an idea than an object, belongs to an entity (person/company/government/whatever), and that entity has control over some of the finer details of how that thing is used. That's even pretty readily apparent in the term itself: "intellectual property", property that is more of a thought than a thing. Maybe it's a bit euphemistic, and to someone with Stallman's beliefs, the propagation of the term probably seems more than a bit dystopian, but it's still a useful umbrella.