Because Kinect isn't an accessory anymore. Its a core part of the system and vision that Microsoft has for the capabilities of the Xbox One. Knowing every consumer has it hooked up and available to use opens up a lot of doors, and with the impressive tech behind it, it could actually be really interesting. Its pushing innovation.That, too. To make such an accessory mandatory is beyond ridiculous. I can't believe that the Xbox One will not even work unless Kinect is hooked up. It's not very considerate of the consumer.
If Sony dont really believe in the Eye that much and dont see it as a core peripheral, then yea, maybe dont include it. But really, why did they even bother then? Its just going to be an afterthought and likely wont do very well and will only receive half-assed support.