You're also assuming that everyone who uses a computer knows how to modify it and not break it. Understand that a lot of Apple users want a device that just works. They don't want to have to mess around with the operating system or type a lot of intimidating commands into a terminal just to change something. They want to go on Facebook, play some games, listen to music, or type term papers. DRM does nothing but prevent things from "just working." It locks down where you can watch DVDs, how you can watch them, how you can use things, and can even lock you out of something if there's a problem on the developer's end (example: Assassin's Creed 2 included a DRM that needed an internet connection whenever you played the game, so if Ubisoft's servers went down or you don't have access to Internet, you can't play the game). To say that DRM is for the user's benefit and totally not for the developer's is insane.
Comparing the MPAA's restrictions with Apple is a bit unfair, and their success is only due to almost a complete monopoly in that area. The MPAA executives look at their audience with dollar signs in their eyes. Apple looks at them with those same dollar signs, but also with care and pride for the products they push out. They want to get a good product out. They adopted new business models like iTunes, iCloud, and even Siri. The MPAA is the antithesis of innovation and doesn't care about the products it pushes out, it just wants money.
Sorry for the long rant, but I needed to do it.