In fairness, DRM is capable of preventing very casual misuse. The DRM on games keeps a kid from saying to his friend, "Oh, let me just copy that for you." If you could have something akin to DRM on guns, it might prevent little Jimmy from shooting himself accidentally while playing with it, and it might prevent a casual street thug with no expertise from stealing it.
But you're right, it won't stop a determined individual with expertise from gaining access. Even at best, you can't think of it as an absolute control over access. No security is absolute. The problem, to my mind, is not the abstract intention of embedding security to control the use of a product or technology. The problem is using security in digital media to restrict the access of people who have "purchased" that media. Specifically, the problem is that the people designing the DRM aren't able to anticipate (and therefore allow) all the possible legitimate uses. If they've sold me a movie, they don't know all the devices I might want to watch it on. They don't know what kind of conversion I might want to do on it 5 years from now. They can't separate the unlawful distribution from a legitimate fair-use distribution. What's worse, many people suspect that the media companies are actually attempting to use the DRM to restrict fair-use on purpose to force us all to constantly repurchase the same media.
So that's the problem. "DRM" is really just security. Security can be good, but poorly designed security will cause more trouble for authorized users than for unauthorized users. Security can also be designed, maliciously, to allow abuse by the designer. In short, the problem with "DRM" is that it's security for a product that I purchased, designed to benefit someone other than me. Putting a car alarm in my new car might make sense. Designing that car alarm so that the manufacturer can (and will) lock me out of my own car whenever they want... is not such a great idea.