In theory, you're right and this is the distributor's responsibility. However, as we've seen with the agency model, even Amazon ends up taking the distribution terms large publishers give them. Most publishers (and, in particular, all of the large publishers) are the ones who insisted on DRM in the first place. They're the ones who are going to end up handling the aftermath.
Speaking of Apple, so far as I know, no one got their DRMed music officially unlocked for free. You either had to pay for an iTunes Plus upgrade or, more recently, subscribe to iTunes Match. That's the sort of thing that I would like to see avoided in the ebook space.
Think of how Apple will now let you re-download every TV Show you ever purchased from them onto your iPad. Likewise, Amazon will hopefully do a deal with the publishers that will let their customers re-download all the books they've purchased (in DRM-free format)