There is good reason to not offer alternatives. There will be platform fragmentation, and end up being a worse experience for both developers and users. Devs probably need to publish on all stores to cover their bases, otherwise users need to know which store to buy from or have to search for an app in every single store.

Also, Apple has changed their subscriptions policy: http://www.macrumors.com/2011/06/09/apple-reverses-course-on...