Most netbooks offer Linux as an option and due to no Windows tax they are usually either cheaper or make up with it with better hardware than their Windows counterparts.
Not even nearly true anymore. Microsoft freaked out about the linux netbook phenomenon and slashed the price of XP on netbooks. Combine that with the awfulness of the early netbook distros, and it was an easy decision for netbook manufacturers to go with Microsoft again.
I'm hopeful the netbook wars are starting again in six months or so but we'll have to see. In the meantime we just have to admit that Microsoft owns that market as well.