By giving you a crippled, nerfed WinRT API to develop to instead of Win32. Apple's iOS APIs are crippled, too, but the iPad is so much cheaper, niftier, and more pleasant to develop for (seriously, Cocoa APIs are pretty awesome). The key advantage of Windows -- backwards compatibility with prior versions of Windows -- is going to be to some significant degree, lost with WinRT. In short, Microsoft is going to have a hard time attracting a critical mass of developers to this new platform without... incentives[0].

[0] http://www.penny-arcade.com/comic/2000/10/23/