When Dell sold Linux laptops before, the problem was always that they treated the Linux version and the Windows version as a separate SKU. So you were buying the Dell Latitude 4100 Linux version or the Dell Latitude 4100 Windows version. You couldn't just go to the Dell Latitude 4100 web page and select Windows or Linux from the Operating System drop-down menu. The regular price of the Windows version would be $1000…
Dell also needs to be able to promote their laptops using features of the OS, or software that comes with it. They also need to mark down the OS versions to coincide with promotions or market activity.
Plus, if you're a linux guy, and the windows version is cheaper, just buy that and get the ubuntu image or drivers from dell support. It's not like canonical is missing out on a sale.