Alternatively, why DOES the PS3 version include Portal 2 for PC/OSX Steam? When has something like that ever happened before?
As far as I could find, there hasn't been any official statement from Valve/Sony regarding this. One might speculate that it was part of Valve mending fences after repeated jabs at the PS3. Or it is part of the deal that lets Valve tie in Steamworks on the PS3.
Whatever the reason, it's a shame the same deal was not reached on the Xbox.