The yearly fee pays for among other things the priviledge of 'renting' the space. Tagging on 15% & 30% to in-app purchases is just an exercise in how to money grab.
I don’t think that $99 a year pays for as much as you think it does. It’s like paying to enter Disneyland and then complaining that you still have to pay separately for food and souvenirs.
I think Apple needs to rename some things.
Instead of calling the $99 fee a "developer fee" and then people are confused what it is... it should be called an "Application Fee" to sell apps in the App Store.
And then the 15% or 30% fee should be called the "Developer Fee"
That would clear things up a bit.
Otherwise... developers will keep saying "I pay $99 a year... why do I have to give Apple more money?"