The publishing industry would give anything to have the option to impose DRM on all online text in the same way that the film industry has for video. Indeed, publishers were so desperate to add DRM to ebooks that many of them adopted Amazon's DRM system without thinking it through. By effectively making Amazon's system the de facto DRM standard, the publishing industry has handed control of the ebook system to the retailer - read this excellent post by Charlie Stross for a full explanation of what happened and what it means.
That experience, I think, is why the publishing industry has not so far pushed for DRM on the Web: it needed a completely neutral DRM standard that would not give control to any one entity. The new HTML5 DRM framework provides publishers with exactly what they need: power over users, but independence from any one DRM supplier.
DRM for video is simply a Trojan Horse for all the copyright industries. Once all the main browsers have adopted it for video, the publishing (and music) industry will be able to point out that extending it to their media will be a small step now that the basic plumbing is in place. By acquiescing in this move, Mozilla makes it even more certain that this will happen.