Apple does "force" you to use the MAS. You can't use the iCloud APIs unless your app is sold in the MAS which means you also have to accept sandboxing. Who knows what else will be tied to the MAS in the future.
The situation is farcical because developers are paying Apple 30% of their gross revenue for the privilege of a sales channel with no customer information and the spectacle of begging for things like upgrade pricing and analytics.
For some developers the MAS is a really nice alternative to direct distribution. You don't have to setup a merchant account and deal with all that. Your apps are automatically searchable (if not really discoverable). You have a built-in review/rating system to establish credibility pretty quickly (which absolutely impacts sales). It's not a horrible system, and again, Apple doesn't force you to use it... unless you want to use iCloud, but then again there's nothing stopping you from doing your own cloud sync solution as Omni has done.
What if they suddenly say only apps distributed in the MAS can use AirDrop, CoreData, CoreAnimation, etc.?
It's not as if they will bother telling developers beforehand.
Go look at the Apple developer forums today, under the iOS 7 beta section, people only found out that the iOS App Store allows older versions of apps to be downloaded because 9To5Mac found out about it and published a blog post. No official guidance (as yet) has been given to developers.
There is no slippery slope here.