Common sense has little place in the business world.
It's a fairly well known fact that Microsoft was under a consent decree that prohibited such things and, while the consent decree is expired now, these practices are still in place. For example, how would you count a Windows license "gifted" from the WinDiv to the hardware guys building the Surface? The hardware guys are simply adding to the BOM the cost of the license and drive it down to the customer. Not doing that would raise allegations of dumping and potentially antitrust issues again (i.e. using a product - Windows - to drive sales of another product - Surface) just like Windows and IE years ago.
While they may have to add Windows to the BOM they can also afford to make zero profit on the device and reap both the "profit" from selling a Windows license and the market share they capture from the OEM (and hopefully Apple). It'll be interesting to see if we start seeing advertising coming out of Microsoft where they do direct comparisons with OEM products they "compete" with. That means it's game on and Microsoft is looking to duplicate Apple's success in the hardware area in addition to (in place of??) selling licenses. If that's the direction it ends up going then Microsoft certainly has an advantage here and I'm sure there will be plenty of lawsuits brought on by such a move.