Apps are not run on the STB.  All applications deployed on the U-Verse platform actually run on an application server in the VHO, and the STB gives you basically a remote desktop session to the app.

If AT&T were to open up their application server to 3rd-party apps, then The Weather Channel could deploy a weather application into each VHO, and then the STBs could access it like any AT&T-deployed app.

However, I don't see AT&T allowing a 3rd-party app unless the 3rd-party is paying AT&T.  Since TWC wants AT&T to pay for Intellistar, I doubt TWC would spend money to deploy the app either.