Hell, it makes sense to release drivers open source anyway. People can keep them up-to-date and whatnot. Less of a load on the company!
You just think that. What will happen is, open-source branded drivers will pop up all over, for Windows and Linux, particularly if they open-source the Windows driver.
As these become more frequent, people Googling will find the open-source drivers. Maybe they'll have heard FireFox is open-source, and they're using it, so open-source drivers must be better!
Then there will be a bug in the driver, and the user won't know how to fix it. The user will try to call ATI, or whoever, and they'll say "Sorry, we don't support it."
Now, NVidia supports their Linux drivers, and regularly updates them. You might see other rewrites of it, but not because NVidia oesn't support their drivers.
So as a business decision to support Linux, they need to decide whether they:
a.) provide a major rewrite to their drivers and keep them updated and supported, or
b.) turn customers away.
Either way it's a major investment, and both are decisions that will cost them in the long term. Because Windows enjoys such a massive market share over Linux, it's not that Microsoft has an evil conspiracy against Linux - it's because it's not worth the return on investment to write to Linux. As you all constantly tout, Linux is used predominantly in the server market, where you won't find Radeon 9800s or GeForce GTX 7950s.
So there you go - there are the business reasons for not going open-source with your drivers. When people say they're releasing a "driver for the NVidia Xxx", NVidia is going to get a phone call when the driver doesn't work.