In the past, you bought software in a box from a retailer who took a margin as high as 50%. After you pay for manufacturing and distribution (shipping shrink-wrapped boxes of CD-ROMs, multilingual manuals and a unique serial number to retailers in Alaska and Singapore is definitely not free) a developer would be lucky to end up with one third of the retail price in their pocket.
The Apple app store is actually a huge improvement over that model; it just seems unfair because digital sales shouldn't cost much, right? Right??
Well, Apple created the marketplace. They spend a lot of money making that marketplace work correctly, keeping their software platforms and merchant facilities robust for customers and developers, and putting that marketplace in the hands of hundreds of millions of people. They built this opportunity for devlopers and every developer who has built an app did so knowing exactly what the terms were.
If you don't like it, don't complain after you voluntarily and knowingly agreed to the terms.