The user experience of in-app purchasing on iOS is very good. Compared to the myriad online purchasing systems I've experienced in over a decade of buying things online, it's easily the most reliable and satisfying I've encountered. Case in point: I've had two separate third-party apps crash or stall during in-app purchase (one because of a loss of network connectivity, another where the app just crashed). Upon relaunching, iOS was able to work out where I'd gotten to with the in-app purchase and reassure me that I wouldn't end up paying twice.
This kind of reassurance is invaluable to consumers who aren't tech-savvy, and will almost certainly lead to higher conversion rates. In fact, it means that the user experience of a customer buying in-app purchase with a completely different product makes them more likely to use in-app purchase with your product! Where else can you benefit from this kind of self-reinforcing behaviour? Apple realises that providing this seamless experience is important, and so does what it can to discourage alternatives where a worse experience (an experience which might lead a customer to erroneously blame Apple or the device) might be the result.
Another important point to remember is that Apple targets the iTunes store to run at break-even (http://www.appleinsider.com/articles/12/02/23/tim_cook_addre...), so accusations that this is motivated primarily by profit are wide of the mark. Yes, that 30% cut helps run at break-even, but let's not forget what other benefits you're getting as part of the deal. And if you're certain those benefits aren't worth it, go try the Android Marketplace, and let us know how that works out for you.
What Apple are doing with the App Store is not expensive for Apple. They hire a few people to run some servers and a couple more to filter apps. It's incredibly cheap and as many of us know you can run a content vending business pretty cheaply as it is. Sure, the benefits of using Apple's UI are nice but they are not, in my humble opinion, worth 30% of every purchase.
To put this another way, I read an excellent article about iTunes (http://thetrichordist.wordpress.com/2012/04/15/meet-the-new-...). Apple takes on zero risk in its iTunes business and nets 30% minus trivial operating costs for vending out some files over the internet. What do you call a business model that asks for money without taking risks? Rent seeking. The App Store and iTunes are both examples of rent seeking behaviour.
The App Store runs at just above break even: http://www.macobserver.com/tmo/article/apple_app_store_runs_... http://news.cnet.com/8301-13579_3-20008540-37.html -- figures of less than 1% or 2% of gross profit. That is not "a killing."
It is amazing Apple somehow spends so much money on reviewing apps and providing the app store that they can't make much money on a 30% cut.