More specifically people who use linux, often don't like windows or Microsoft, so donating what $500 to this company for no good reason..? Don't kid yourself. If you doubt it, offer 2 or 3 random customers a $500 discount for the laptop without the windows licence. See how many don't care either way.
I'm not sure where you're getting your figures from, but I'm betting they're wrong. Let's say I'm Sharp or Dell, and I'm pushing 10,000 laptops a month onto the streets in the US. I don't know whether that figure is high or low, but it sounds good to me. Do you honestly think Microsoft is going to charge me $500 to install Windows onto each laptop, when I could just walk over to CompUSA and purchase the full copy of Windows XP for $200?
I don't know the exact figure, but this Windows tax is smaller than people make it out to be. They are not donating $500 to Microsoft.