I have always assumed that publishers embed an ID somewhere in the Mobi or EPub files (they are just HTML, you could insert all sorts of things that would never be displayed) since it is trivial to remove the DRM from them.
"The average consumer will never notice," said the music industry.
I resolved never to buy music with this feature.
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Unseriously: If they did this to Finnegans Wake, would anyone notice?
Seriously: Screw this.
http://en.wikipedia.org/wiki/Coded_Anti-Piracy
Of course, movies and books are probably opposite ends of the spectrum when it comes to artistic integrity.
As far as artistic integrity goes, it makes sense that the immense cost of a film being covered by the studio gives them quite a bit of say in the filmmaking process, for good or ill.
Of course the only payoff is that pirates now need 2+ copies of the book so that they can't be identified. You can push it harder and identify more words to get the pirate to buy more books but the return (in books bought) becomes much much worse and identifying them becomes more infeasible.
Having bought a lot of e-books, I've yet to come across something that would qualify as "good". Sloppy, careless conversions are the rule rather than the exception, sometimes to the point where I ask myself whether a human being has actually read the converted mess.