You could very well do that. You could also crack open the kinect as soon as you buy it and change it so you can disable it by a switch or something like that. Although permanent disconnection apparently isn't a good idea as saying "xbox on" will I hear be the only way you turn the device on.

Really though I think it'd be easier for them to just give customers the option not to plug it in, but my guess is that they're trying to make sure everyone uses the kinect that they ship with the console (its not necessarily being used for nefarious purposes), but with that being said I still don't trust Microsoft.