Yes, DRM developers lie to us and claim that this is about stopping infringement. But that's only a pretense. They know that it only impacts non-infringing users. But they like having more control and power over the non-infringing users.
* What devices and equipment can view it
* Time limits on watching
* Regional lockouts
* Time shifted viewing
The movie industry fights tooth and nail to "manage" our rights, not just with DRM by plain RM. In NZ for example there is a ban on parallel importing films for a period after release! Here is something interesting: http://www.parliament.nz/en-nz/pb/debates/debates/50HansD_20.... Choice quote: "The 5-month ban will effectively give cinemas a further 1 or 2 months to exclusively screen films without competition.". Feast your eyes on the pandering! There is no "transition" period going on. It's the movie theatres using laws to prop up their new business model of stealing 20 minutes of your time and selling it to advertisers, after taking 20NZD admission.
I think your point is exactly correct, DRM does little, if anything from stopping illegitimate actors, but it does serve to manipulate and control legitimate users (for example preventing them from using the DRM content on certain devices, thus requiring them to potentially purchae it again.)
I think it's a much simpler explanation that companies really do hope that DRM will reduce casual illegal copying and sharing, even though they know it can't absolutely stop determined people.
The lock is a tool of the user against third-parties.
Doctorow's Law: "Anytime someone puts a lock on something you own, against your wishes, and doesn't give you the key, they're not doing it for your benefit."
A lock you have the key to is nothing like DRM.
kmonsen is making the point that door locks on homes don't actually stop determined burglars. The only reason they work is because most people are relatively honest and aren't willing to take the step of breaking the lock (however trivial that may be), even if they'd otherwise happily open an unlocked door and walk in.
Not only that, but the fact that a door is locked almost certainly has legal implications for trespassers (though I don't know if that's really applicable to house door locks as much as it might be to locks on doors leading from public areas of commercial buildings to private ones).
My suspicion is that they have contractual agreements with the non-technical folks in the studios that they have to "encrypt" content, and the technically-competent redistributor has no direct interest in the crypto being sound. If the API gave them 256-bit military-grade AES encryption, but only in ECB mode, they'd probably use it.
I have a hard time seeing how implementing DRM provides any value to media companies, other than a false sense of security.