When the concept of a netbook was initially brought to market, it was usually pitched as a very portable device that was able to remove much of the cost involved in hardware and software. The software costs, for instance, were supposedly largely offset by adoption of Linux desktop distros to power them. The Eee PC was one particularly pitched as Linux friendly, and the assumption made was that people would opt for a lower cost Linux machine as opposed to a stripped-down Windows XP.
That's not the case, though, and in fact is far from reality. As it stands, Microsoft sees just about the same adoption rates on netbooks for Windows that they do on desktops. When you isolate netbooks, the software giant has about a 90% market share with Windows XP. A lot of this was assisted by them offering a discounted license fee for XP on netbooks, accompanying it with a set of certain limitations on things like hard drive size. Even though Microsoft's profit margin for netbooks isn't terribly high, they are probably more than happy to take than to take nothing – as it still gives them control of the market and a lot of influence on netbook vendors.