I've heard some of the most ridiculous things in these comments.. including all the bad car-gas/printer-cartridge/dvd player-dvd metaphors. Unfortunately they all have a very important thing in common: those products are meant to have their corresponding 'parts' exchanged by the average user regularly, and the 'parts' are meant to be exchanged quickly and easily. An OS is usually only a one-time thing: you install it and you're done.
The point is, it's not practical at all for HP to not sell computers with Windows pre-installed, for the sake of the consumers. The case could be made that other options should be available, but computers that come with Windows will always be mainstream.
But but computers without an OS forced on you are cheaper and give you more options. And they work! You just have to install the OS on them for them to be useful, but ask anyone who knows anything and they can tell you that's easy...
Show me an entrepreneur or CEO that's willing to license, obtain, and have Windows installed on every computer they buy themselves, and I'll show you a fool. For those who just need what's mainstream and what's best for them and/or their employees, preinstallation is faster, cheaper, and easier. The rest of us linux nerds? We'll just have to do our own thing.