After all, if Apple wants to dominate in this space (and I believe they do) then I have to believe the actual public API will be much, much more powerful then a bunch of lightweight UIKit inspired layout classes.
They've already got partners providing javascript-driven "applications" for the AppleTV (ala iPhoneOS 1 - "web apps are king!"); I've no inclination to believe that they're going to make public an API that provides for code execution, especially in this environment. See, not only are BackRow appliances not applications, they're not run in separate execution contexts either. The primary/only application on an AppleTV is AppleTV.app, and all appliances are loadable bundles (on AppleTV OS < 5.0) or simply compiled in (>= 5.0) and run in the same shared context.
That alone makes binary bundles extremely dangerous. It also means that a full re-architecting of the system would be required before third party applications were supported.
Because I don't doubt they want to (and to some extent have the potential to) dominate this space, but I don't think it's necessarily valid to assume that means opening a public API.