Say you work for Gamestop and someone bought a copy of World of Warcraft, should Blizzard have to pay 30% of each monthly subscription charge to Gamestop?
If someone buys a WoW game time card from Gamestop, Gamestop will take a cut of that. Last I looked, the WoW software and expansion packs cost nontrivial money, and Gamestop will get a cut of that too.
If the WoW software cost $0.00, and only sold game time directly from their website, do you think that Gamestop would, out of the goodness of their hearts, continue stocking, promoting and supporting the software without any income stream?*
The 30% cut Apple gets is not just a donation to the Steve Jobs Memorial Fund: it pays for Apple's online payment processing (clue: payment processing costs serious money) access to customer's iTunes gift cards, and avoids the need for customers who are already signed up to iTunes to re-enter their payment details (clue: having to faff around with credit card numbers could derail what was otherwise an 'impulse buy').
Of course, what it is also doing is subsidising the 'service' that Apple provides by offering free apps - or, to put it another way, the free apps are a loss-leader to pull in punters who might go on to buy paid apps. Wouldn't work for Gamestop because the overheads of giving away free stuff from a brick-and-mortar store are orders of magnitude higher than giving stuff away online. It wouldn't work online if everybody turned their paid app into a 'freemium' product that cut Apple out of the deal.
People who think Apple's cut is unreasonable don't seem to have much idea about commercial realities.
Google/Android are in a different game: Google are fundamentally in the advertising and data-mining business, and the whole Android project seems to be a loss-leader to ensure that people use their search and mail services.
*actually, in the specific case of WoW it might just work if people bought sufficient shedloads of toy^H^H^H collectables to make distributing the game a valid loss leader. I doubt that there's a big trade in Microsoft Skydrive action figures, though.