Windows is still the universally supported o/s, whether it's any good or not is beside the point.
All built PCs should come with it preinstalled, what else are you gonna give them? Linux (or version of)? Most people don't even know what it is, let alone care. But sure, the option could be there (as I'm sure it is from many online pc sellers), not that anyone would take it up (for the most part).
It should be a clean install of windows though, no antivirus/video/audio/blah rubbish which will slow everything to a crawl.