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.
And Samsung would simply reap the profit of selling RAM, display and other components to Microsoft. That profit would probably compensate for the Windows license that they have to pay for, so in the end it would be a wash.
That is, if Samsung manages to produce !crap for once.