Probably because the consumer level kit is full of trail versions of software the suppliers have paid to have pre-installed in the hope enough mugs will pay to "upgrade" to the full versions.
The reason according to my friend who works for Dell is rather more mundane. The average purchaser isn't all that bright when it comes to computers; heck, we've all had a few laughs here over some of their misunderstandings. So, average punter purchases the cheapest machine available. Note here that charging more for a machine without Windows + bloatware would not be smart marketing. Average punter then attempts to install Open Office that he/she/it [delete whichever is inapplicable] just purchased for $AU18.95 from eBay, but can't because there's no OS installed. Result = one very pissed off customer.
Requiring customers to specifically request there be no OS keeps those who want Windows and those who don't happy. Except for those who are too fucking stupid or lazy to pick up the phone, or use Dell's chat facility to make the request.
Biting the hand that feeds IT
