I may be simple-minded but I would have thought that people either want to read a book or they don't. But now is there really a market for people who want to read a book but only if that book's content is made different from what it actually is? (I'm worried if that's the case).
On the other hand, I fully understand that the aim of a publisher is to sell more.
But what the publisher did in this case was not like that; these modifications weren't designed to make the books more accessible to people with low reading skill ('fat' is not a hard word.)
People have been willing to pay for this kind of thing for a long time.
It's a little different now in that it's the original rights-holder doing it but money talks and why argue over fair use when you can do it yourself?