Because it's not like fuel. Most people have a fairly inelastic demand for fuel. You kinda have to drive to work. You really don't need apps, and no one needs to make the apps. Running sales can increase consumer demand. Increasing the profit margins for developers could increase supply of quality apps, thereby increasing net revenues for Apple on a smaller cut of the pie. I rather like the current system for it's simplicity and while of course I'd like them to charge less I think their cut is reasonable.
To gain more development support. More developers = more apps = more users = more revenue.
It makes sense.
There's certainly a tipping point though. Having the store flooded with more low quality apps doesn't help them, and they seem to be doing pretty well currently.
well even if they take 50% cut, developers are not going anywhere else. So, the equation is more like whatever cut apple takes = more developers =...= more revenue
Apple's not the one hurting for quality developers.