Since its launch, Google Android has been compared with iOS, the mobile operating system by Apple. This comparison makes sense, but they are two completely different initiatives. And one of the points where there are more differences is precisely the app stores, something weird to many people: Why Apple has only one app store and everyday there are more for Android?
It is important not to forget that Google is a company that gets 96% of their income through advertising, and Apple is a hardware company that obtains over 90% of their income by selling different devices.
The Apple AppStore is not intended primarily as a tool to generate revenue for Apple, but as a natural extension of the hardware. What worries Apple is controlling the user experience of their customers to ensure their satisfaction and, consequently, to sell more devices so that they complete their "collection". What is important is cross-selling hardware and its continuous updating. In case they get AppStore revenue, it will be just a little extra. It currently represents a percentage in the income similar to what Apple gets from the sale of software or peripherals. See link above.
In Google's case, the important thing is advertising. In this particular economic field it is the leader in web by far. And Android is a bid to extend Google's advertising kingdom towards the mobile domain. That´s why its CEO said recently that his rival is Microsoft´s Bing, not Apple. The company wants to dominate the mobile ecosystem via its OS to dominate advertising.
With this in mind, you now understand Apple may only allow one exclusive shop. But Google not only is not worried about the proliferation of Android app stores by carriers (Verizon, Vodafone, etc.) or retailers (Amazon, GetJar, etc.); Google think it very likely will help to expand Android market share in mobile, which is their true goal. No matter where a person buys their Android applications: what matters is that they have installed the operating system Android.
So we shall see more and more Android app stores. And Google will be happy because they will help them to achieve its target.