a) they don't actually get 30% (they get what is left over after giving the developer 70% and then subtracting their fixed costs... small money transactions is considerable in fees, bandwidth for them is cheap but not free, etc.)
b) it is now public (due to the Oracle case) that the Android division of Google actually lost money in 2010
c) despite their massive volume, Apple's App Store makes sufficiently little profit that they don't even break it out as a separate line item on their 10-Q (instead lumping it under "music related products and services", the same category that handles both iTunes and iBooks)
To expand on #C, it is seriously my understanding that Apple manages to only barely stay in in the black on the whole App Store model: their goal is ecosystem control and a competive advantage for their hardware platform, which in turn is where they make their real money.
Looking further at Apple's business model, you get ~$170 margin per iPhone sold; I know many people who have owned 3 iPhones over the last 5 years, but absolutely no one who has purchased the $1700 in apps that would be required for even the full 30% (that Apple can't possibly be keeping all of) to make that same amount of money.
In essence, you just can't subsidize a piece of hardware like this with app sales. I mean, this is going to be ludicrously obvious if you just take the individual device: if they wanted to subsidize it by even $100, and even if they managed to keep the full 30% (and again: they can't and aren't), then the average tablet user would have to purchase >$330 in apps per tablet... that is ludicrously unlikely.