It's very simple: back in the old days, when there was no monopoly, many chip manufacturers gave away thick, printed databooks. It was expensive for them, but they needed developers to use their products.
Now, however, there is a monopoly. You don't need to attract developers. The only concern is to have a driver for Windows, and having that driver included with the Windows install disk, so that your device, be it a soundcard, graphics card, or whatever, is "easier" to use. I wonder if there is a "dark hand" behind it...
Some years ago I sent a proposal to the European Comission: banning the sale in Europe of peripherals for which there is no public interface information available. It should not hurt the manufacturers, as the information can be made freely available in Internet (it's cheaper than shipping huge printed manuals), but it would have a side effect: the driver advantage for Window could disappear.