It's not as simple as just blaming gnome/Ubuntu, it's more the end user. More so, your right when you say that win uses aren't used to having this choice. It's not so much the distro not advertising a choice, because all distro's have set defaults, but more so that the users aren't used to having that extra choice. Windows mods are much more intricate than linux, by far, so they aren't used to a single command giving them an entirely new desktop option.
If you think about it, that's a really powerful command. That's almost one of the most powerful commands you can do to a system. With a single command you can change the entire GUI layout, creating a brand new look and feel. You don't have anything like this in windows, or any other OS (outside of the whole *nix family).
This is why the *buntu branches were made, because people don't understand that you can maintain and use Ubuntu with another desktop just as easily as Kubuntu, Xubuntu, ect... does. It's the same thing, well almost. Generally it's not until you've had more experience can you look back and then realize the wasted effort switching to Xubuntu from Gnome just cause you want a new DE. Just grab KDE, no need to install a new system.
You can also view it that windows users are trained to think they have to re-install the whole system to get a new feel. Win98 -> XP -> Vista -> Win7 This is done whether or not there are real system advantages. They just think "Oh, if I want the better system I re-install a new OS". This not only trains them that installing a fresh system is better but in order to get a different UI you need to do this. This of course isn't so, at least not in the *nix world.
to you and i and others on this forum the knowledge of "sudo <your chosen installer> <your chosen installer install command> kde/xfce/openbox/etc" should be common knowledge to that of all linux users, not just us who choose other-than-a-*buntu system.
i'm not arguing against defaults here, i do not want to download a 4.1 gb iso dvd of any distro just to get kde or gnome, or other de/wm all packaged into one with a "choice" at install, but at least give the idea freely and openly, don't come out with "k*distro" "x*distro" "L*distro", and then basically "adopt" them as variants, and let ppl continue to believe they are different projects than your "default" distro.
tell them that gnome is the default DE, but with this command they can easily get xfce, kde, etc with it.
i like ubuntu, i used it for a good many years, i didn't leave ubuntu cause they decided to switch to "unity" or even cause there are more *buntu's than pair of underwear i own, my ideas and direction clashed and i left them pretty much on a purely philosophical POV.