Since this isn't really the place for extensive political theory, that doesn't seem very relevant to the point, so I deliberately didn't go into a digression.
My point is that there are two views of property: it's a right that is somehow inherent/moral/pre-existing; or it's a convention that exists for its benefits to society. How we can know whether it has benefits, and who decides whether the convention should exist, is a whole different discussion.
I don't mind diverging into a bit of political theory here, since it seems important to the topic.
I'd say that if we're going to view 'society' in a meaningful way - that is, not merely as a conceptual abstraction, but as something exists empirically - then we have to acknowledge that society has a definite nature, emergent from human psychology, emotions, physiology, neurology, etc.
And if we accept that society exists, why should we not recognize that the recognition of things like 'rights' and 'property' emerge from the very same antecedents? These concepts are all descriptions of fundamental relationships between people and other people, and between people and their physical environment.
The idea that social conventions exist for the benefit of society is something of a tautology. Obviously, anything that anyone creates or does is something they're doing because they've judged that it will be of benefit to them. The ideas of 'law' and 'government' follow the same rule - these are also just conventions that people established, in the expectation that they will somehow benefit.
But the real question, the empirical question, is this: what are the actual motivations that have led people to establish these conventions? I suspect that the idea of law emerged after the idea of property, as people sought to maintain the relationships they established with physical objects in the context of their relationships with other human beings, and sought a structured and formal way to do so without violence (or perhaps merely dissension and group harmony, given that cooperation is as essential to our nature as competition).
Fast-forwarding to today, with all of that in mind, I think it's reasonable to make a distinction between the conventions that arose from a 'natural' social context and those conventions which are the product of positive law, the latter being a kind of 'secondary' experimental thing, intentionally designed by the mechanisms of the earlier first-order convention.
In my mind, 'theft' is a violation of the first kind of convention, while 'copyright infringement' is a violation of the latter.
Interesting use of the passive voice - who's doing the judging?