Making closed drivers is the companies' way of protecting their products and hindering open source. It pisses people off, but since the driver at least works it stops them from complaining too much.
Albertlee, it is not as simple as just copying some .dll's from windows over to linux to get windows support. The Wine project (http://www.winehq.org) is working on it though, building windows libraries and API's from scratch.
I'm sure that Sun had to be very careful when they made that system, and put a lot of money into lawyers' pockets. The terms of the GPL license, which most open source software is released under, are pretty complex when it comes to shared libraries and stuff, and what they can and can't be included in. See, normally if you use any GPL code in a project, the entire thing must in turn be released under the GPL or a GPL compatible license, but when you are just including a library things become messier.