Android allows self-signing, though. The biggest use case of Android's signatures right now is to make sure only the packager can serve updates (and gain access to the data stored within the sandbox). You don't even need to provision any developer specific signing certificates to install these apps.

In theory Android could start using certificate verification, of course, but right now that's not being used.

That's true. I hope with proper sandboxing Microsoft will allow self signed certs for such apps. I can understand the risk when an application can access literally everything of the user's, but the level of trust needed for sandboxed apps is less.