Second-screen application developers generally write to the device. With SmartGlass, Microsoft is asking app developers to do something slightly different, namely: write to a platform. But Microsoft is not asking just anyone to write this way; only those partners to its game developer network who have permission to do so.
The larger point may be that SmartGlass, which the Seattle software giant made generally available three weeks ago, at about the same time it released Windows 8, is itself both an app and a platform. That is one of several complicated aspects of this Xbox-related software. Another is its genesis as an app on the Windows Phone.