It doesn't matter if alternative platforms are less expensive, if no one buys anything on them.
If the iPad is selling tens of millions of dollars worth of books for Amazon, then Amazon are not going to shut that down in a hurry.
C.
1. Amazon has said that 20% of its sales come from its various apps, and I think it's reasonable to imagine that the iPad and iPhone apps would account for a lot - maybe a majority - of these purchases.
2. If the new policy means that Amazon has to charge the same for in app purchases as out of app purchases, and has to give 30% to Apple, Amazon would undoubtedly pull the Kindle app, as they would be losing money. This would be bad for Apple - see below.
3. If the new policy means that Amazon and Apple somehow split the 30% commission for sales made on the iPad, but prices remain the same in or out of app, Amazon may or may not pull their app. It depends on the details. But Amazon is a low margin, high volume seller (4Q profits were $416 million on $12-$13 bln in sales), so there may not be a lot of flexibility. And this might also depend on whether B&N (and to a lesser extent Sony) go along with the change.
4. If Amazon can sell its books at 43% premium through the app, I would imagine they would keep the app on the ipad. Although I doubt that either Apple or Amazon would make much money through in app sales.
5. It would be bad for Apple if Amazon pulled its app. Amazon sold 8 million Kindles in 2010 - not iPad numbers, but very respectable. Kindle has a different demographic from iPads (its owners are slightly older, slightly wealthier, and read a lot of books), but there is some overlap. Many iPad owners also own a Kindle, of course. If the Kindle app is pulled, Apple will likely lose some millions of customers or potential customers, especially for the iPad. Few, if any, Kindle owners won't move from the book format with 80% of the market to an incompatible book format that shares the bottom 5% of the market with Sony and all other e-readers that are not the Kindle or the Nook. So to the extent that they are interested in a tablet, they will look for one that they can read their books on. And the same analysis is likely also true for Nook readers as well.
Maybe Apple has something else up their sleeve. But as presented, this looks like a change in policy that is bad for iPad and iPhone owners, bad for booksellers, and even bad for Apple.