The difference is that apps are in competition with each other and the app store is more like a marketplace for developers. There is no physical interaction between developer and buyer.

Uber could never call itself a "marketplace" for drivers. It operates much more like a driver agency.

Edit: there are genuine online booking agencies for drivers in London, like http://www.kabbee.com, which act as a front-end for independent minicab drivers. I think this is much more that way it should operate if the drivers weren't employees, as Uber argues.