They need to prevent any actions that would undermine the security of their system and also they need to protect themselves from the user demands that can arise when someone has an incompatible hardwaresetup.
As reported by Light Reading, Cisco, Humax, Pace, Technicolor and others are working on hardware for a new multiroom setup based on a central gateway (or maybe a Cablevision-style network DVR) that streams video to IP-connected extenders called the XI3.
Comcast isn't opening up its set-top boxes (UI and all) in the way we'd like, but it has established a reference spec (RDK) for potential hardware partners to build their own versions of its next generation setup.