The irony here is that many of the folks on this site fully understand how some of this works and the value Apple brings to Kindle and Sony.
1) Amazon takes a 30% cut of books they sell for download to their Kindle. You are saying Apple should not get the same revenue for providing the same service? Maybe Amazon/Sony should take the price cut since Apple is the one providing the Credit Card service and marketing the app through their App store (which is WAAAAYYYY bigger than any of their markets).
2) If there had not been a Kindle App for Ipad, I would NEVER have bought a Kindle book from Amazon as I would never have bought a Kindle reader and don't like to read on my laptop. I have no stats (since Amazon doesn't release them), but I would be shocked if they don't sell almost as many Kindle downloads for Ipad as they sell for their own Kindle Readers.
3) To they guy who asked about Skype credits being bought through the store. Funny example, I wanted to try that out the other day, but I opted not to set up a skype credit card account and so I did not buy any minutes. If it had been an in-app purchase, I would have bought it in a heartbeat. Instead, they got none of my money.
4) To the guy who mentioned giving away games for free and then doing in app purchases - Apple takes 30% of in app purchases as well. That is the whole point. Then I don't have to screw with setting up an account with that manufacturer to buy their .99 doo-dad.
5) Aggregated services like this are the reason I use Amazon/The App Store on other large sites. What would it be like if 50 sites were offering iPhone apps for sale and I had to sign up for each of them to buy some exclusive app I wanted. Sometimes I find stuff a dollar or two cheaper than Amazon on other sites, but I buy from them anyway because I have an account and Amazon prime. Same thing goes for in App purchases.
6) People would buy more products from Kindle and Sony if they were handled through the itunes store and their revenue would go up. It becomes an impulse purchase. I work at a company that sells downloaded software. The #1 barrier to purchase is account set-up.