More than likely, because switching to Linux also drops all the "paid pre-installed windows software" (companies paying Dell to pre-install their software) then this jump in price would give a hint as to how much Dell receives in payment for pre-installing all those windows extras.
Does Dell offer a "clean" version of Windows for the extra price? I can come up with competing reasons for why they would/wouldn't do this.
If you're not spending a huge amount on laptops, though, I can't imagine this being an option they want to offer.