Like stevo said, SkyGo is free if you have a subscription. They've already separated delivery of programming from their boxes. It's only logical that they continue in the same vein by allowing casting. If the BBC can do it for content they broadcast, so can Sky. There's nothing forcing Sky to do this, but I don't understand them allowing tablets and phones but not casting.
Logically you are right after all you can watch Skygo on your tv through a pc or xbox360, however when it comes to mobile they doing all hey can to ensure it stays on your mobile screen and no further