Nope. They are taking a 30% commission for handling payments + quality-testing, hosting and serving the apps and their updates. But most of all for giving developers shelf-space in an online store (which they have build, launched and marketed) with millions of visitors that have entered credit-card info and are able (and willing) to 1-click purchase.
Furthermore they promote and market a large number of quality apps through the store as well as through their own marketing channels (TV, print and outdoor commercials, Apple In Store, Facebook, Twitter, Demo units etc.).
On top of that they've build great (from what I've heard) development tools and are serving a huge developer community.
In other words they offer a lot more than being a 'payment processor'. And their business practice is normal. Just ask Wal-Mart, Best-Buy, Groupon, Playstation or Amazon how their system works ...