I believe you are exactly backwards. DRM is about limiting the user. The GPL is about freedom for the _user_. As is pointed out ad nauseum every time a GPL story shows up on slashdot, the restrictions only apply on distribution of binaries.
Simple example: Once upon a time, in my linux n00b days, thanks to the GPL, I used to change the source of ... what was it, scp and grep? to make the flag for directory recursion lowercase r, because I couldn't be bothered to keep track of what used -R instead. Now, say Apple doesn't release the source for Darwin. It's BSD licensed, of course, so they don't have to. Then, to keep everyone tied into the Apple experience, they implement some kind of hash check before anything gets executed (sounds like DRM), so even if I reverse-engineer and compile my own binaries, I can't run them.
One way, I, the user, can do what I want. The other way, I can't do anything I want, despite that they are both open source. It's not that hard.
That said, I generally agree with you about RMS. I believe his difficulty is that he has valid practical considerations that, when stated in legalese or abstract ethical notions, sound crazy. Then, his unflinching, uncompromising nature drives him into the fringe.