The entire question of what it means to "share" a message can be boiled down to "how do we define a receiver?" This is the other half of analyzing a Digital Restrictions Management system.
In this essay, I've very carefully defined almost every component of the communication model. The receiver is left undefined because it's actually very hard to tell who is the ethical receiver (or, for the computer folk in the audience, the logical receiver). It's usually pretty easy to point to a physical receiver but that is often not the same thing.
Most media purchased from commercial retail outlets are essentially sold to households, not people. I may have bought a DVD yesterday, but my wife or children are free to watch it, even if I am not around. So while I may have physically acquired the DVD, it is clear my entire household has received it in the ethical sense.
This obviously doesn't bring us any closer to a rigorous answer of what a "receiver" is, for what is my "household"? Is it just the legal adults legally registered as living there? Who on Earth can enforce that sort of thing, anyhow? If I have a non-family-member housesit for me, is it a crime for them to watch one of my DVDs? The more you try to narrow down who the "receiver" is, the more very common exceptions you find.
Right now it isn't a crime anyhow, because they could borrow it and watch it that way, so this can be considered as a form of "borrowing". But what of a DRM'ed movie, backed by the DMCA, that I paid for for only household viewing. Would it be a crime for my housesitter to watch that movie because they are fraudulently identifying themselves to the movie sender?
In fact this theory does not point toward a way to rigorously define a "receiver". It points to some ways how we could do that, but it does not prescribe one.
If one tries to narrow down what a receiver is, one is invariably drawn towards extremes, because the lines are fuzzy and people will abuse them. You can't define it in terms of "household", because that's a virtually meaningless term. Same with "family". One is pushed towards either a very narrow view, where only single people can be legitimate receivers, or a very broad view, where entire large, amorphous groups of people at a time are the receiver, as either the restrictions grow, or the group definition grows.
Clearly current DRM technologies are all moving towards the "single person" definition, where one person receives a message, and they may essentially never share it with anyone ever again, possibly not even with themselves if the message "expires". There's no grounds to call this inherently unethical, although the technical difficulties of restricting receivers this much while not restricting their Free Speech rights or creating an asymmetric relationship may be insurmountable. In fact I believe them to be, but this technically is an engineering issue and it is possible that with enough decades of work, a fair but restrictive DRM system could be built. If they are truly impossible then this isn't really an option, ethically, and there is also the question of the damage done while inferior systems are in place, which will certainly be for many years.
On the other hand, we are currently living in the world where the definition of "receiver" is very, very loose, and I'd submit for your consideration that the economy has conspicuously failed to collapse. Right now, practically speaking, I can copy a CD and give it to a friend. Nobody is seriously claiming this is destroying the music industry. (The music industry certainly talks about this on occasion but their real ire is saved for the P2P sharing systems, more about which in a moment.) Is there anything terribly wrong about sharing a movie with your parent, or a friend? Especially if one could develop a culture that still encouraged paying for it? (Anecdotally speaking, I think we live in that culture already. Certainly the purchase rate isn't 100%, but one questions who got the idea that such crime could ever be stamped out, or why these crimes justify restrictions on the rest of us.) The sheer mathematics of life and time work out such that even if giving copies of movies to our friends was as easy as pushing a button, we have better things to do with our time then worry about that.
I think the maximal value to society is obtained with this fairly loose idea of "receiver" that we inherited from older technologies. This includes a seperation between the idea of "sharing with one's friends or family" and "sharing with the world"; we did not condone large-scale piracy in the 1990's and I think that the modern equivalent, large-scale anonymous file sharing to all comers is also unethical. But here I think we can draw a relatively sharp line between "sharing with all comers" and "sharing with just people you know".
The tension here comes from the difficulty in defining "receiver" in different ways for different technologies. Certainly the large content providers of today are comfortable with locking down to individuals and grudgingly admitting that families can probably watch the same movie (though they'd make them pay extra if they could), but that's not necessarily the social ideal. And one must also question the costs of implementing these schemes, which are expensive in both development and maintenence costs, vs. any gains for anyone, even the actual sender, let alone society. Unfortunately either a restrictive or a free model must be chosen, and as usual it will be difficult to impossible to mix the two.