The (traditional) publisher actually does some fucking work! For a start, he will get it laid out so it looks nice, proof read, offer advice, provide illustrations, support and marketing of your book, in return for (sometimes) upfront cash, other times royalties or a combination of the two. They may even buy a series of your books. Most importantly they will market them and try and get them in prominent promotions either on-line, in store etc etc. However, they take a risk with their money. (hence why they are so selective, though that one that missed harry potter - doh!)
Apple, by contrast, give you a word processor, you do all the hard work, formatting etc. You will need to promote yourself, and do all the other stuff. If you are lucky enough, they will give you a prominent position in the bookstore, but then they most likely will not. Either way, they get 30% for *you* doing *all* the hard work. And then, you may do all the hard work and they may just decide to tell you you can't sell it, they don't like it. (Odds of a book "How android is better than ios" ever getting published are not high :P, but i suspect "Taking old people on a one way trip to dignitas" equally won't be allowed )
That is why it just looks a bit unfair. They get 30% profit for 0% risk, unlike a traditional publisher. You take 100% risk for a potential 70% share. I know which party I'd rather be!