Not quite. I don't know book margins, but Amazon doesn't make books. It sells them. Apple is taking 30% of the cost, but that comes out of Amazon's profit, which is less than the sale price.
So, in your example:
With two ten dollar books, maybe only $5 each goes to Amazon. In a direct sale, Amazon is making $10 total for the two books, and they have to pay the other $10 to the publisher and author, etc. for the actual materials itself. This gives Amazon a 50% margin (this is just an example. I don't know Amazon's actual cost of goods sold).
With the 6 sold via iOS, Apple gets 30% of each sale. 30% of $10 is three dollars. That means Amazon is only netting $2 each book ($10 - $5 cost of goods - $3 apple slice). 2*6 = 12, or only 2 more dollars than the two they could have sold on their own. Meanwhile, they've had to spend resources to sell those books on Apple (building and maintaining an iOS app, paying the account executives who work with Apple, etc.). Your example ALSO assumes that they sell 4 additional books because of the Apple platform. I think it's more likely the only sell 2 additional ones.
The real problem with this idea, and why Amazon and Netflix won't do it is this whole process reduces their profit margin. Margins directly translate into stock price (look up price earnings ratio). Amazon, a public company, could see their margins slashed if sales really shot off on the Apple platform, if Apple gets 30%, even if their total sales went up (i.e. if their margins on two $10 books was 50%, their new margin in the made-up-example above would be 12/16, or 20%). Lower margin business have lower P/E ratios. Even if sales goes up, if their margins go down enough, their stock will fall, and they'll have less ability to go to the public markets for growth capital, and some investor will rise up and try to win control of the company to take it in a different direction.
I can't say this clearly enough -- THIRTY PERCENT WILL NOT HAPPEN. Amazon, Hulu, Netflix will all walk. This is not Apple trying to make more money, this is Apple pushing these folks off their platform. Period. It's a punitive, ridiculous amount, and there is no way these other companies will pay it. An original content producer might pay it (because Apple then becomes their channel instead of Amazon), but not an existing reseller/channel player like AMZ.
People are defending Apple as if they are sticking up for the consumer. They are not (in this case). They are trying to lock down their platform for their content channels, and scare away the other channel players. This is about less choice. It's a logical move by Apple, but I would submit they are mis-playing their hand. The iPhone and iPad are not dominant ENOUGH to make this move. I think they will get burned, and they will back-track from this for the the near term.
