They make preinstalled Linux systems. From a cursory glance, they look reasonably nice and the prices aren't crazy (although my perception of 'non-crazy price' may differ from that of a U.S.-ian.)
How big is the market for pre-installed Linux systems? I would buy one if they had decent distro selection. Being based in China I wonder if I should start an assembly shop for export. Custom-designed server business also appeals... but too high churn on parts to be possible as a now-and-then business.
I don't know - the fact that this is the only manufacturer I know of that specialises in them would indicate it's not huge, and the kind of people that run Linux are the kind of people that generally want to build their own computers.
If you're thinking of starting one, I would say absolutely avoid the desktop market. We can already build our own custom Linux boxes. What we can't build is a solid, reliable, well integrated high-end laptop. With the MacBook Pro continuing its fall from grace as the go-to developer machine, I'd expect you'd find a ready-made passionate (if niche) market.
Mainland Chinese RRP for bottom spec (1TB/8GB) Dell XPS15 = 9,999元 = USD$1,445 = EUR1,368. Might get a volume discount. Add your preferred Linux distro, test and ship internationally, say USD$1600 or EUR$1500 (plus optional rush shipping), would anyone buy it? US prices are already at USD$1,450. Sounds like zero margin, bad business.