Why not? It's your hardware and you can do what you want with it.
Apple have never complained about people installing Linux on MacBooks. In fact back in the G4 days they turned a blind eye to Mac on Linux which let you virtualize OS X on top of Linux (at about 95% of full speed!)
They discussed it in their shareholder's call. But really though, is it that surprising? 30% does not go far when you take servers/bandwidth, customer support and credit card transaction fees into account selling mostly $0.99 products.
except that they don't have CC transactions for EACH app you download, do they? I believe they make a bulk charge at the end of the day so if you download a $2.99 app and three 99 apps, they don't pay a transaction cost for each app. They pay $.25 + 2.9% of $5.95.
They may break even on the numbers, but the apps are one of the biggest features and selling points for an iphone. Apple even knows this too, when was the last time you saw an iphone commercial that didn't say 'there's an app for that'?