Yeah, coming from the RH/Fedora world, it seemed weird when other distros starting spinning specialized versions for a different window manager on the desktop, such as kubuntu. I guess that's because they wanted to have more control over the "experience". RHEL is about getting shit done when you know what you are doing, not holding your hand and making you feel comfortable, so I guess with those different expectations it's a bit easier.
Did they always? I remember it usually being as simple as installing the RPMs, and running the switchdesk utility. I always had to do that (or generally something more involved because of my choice) to run FVWM2.
Ubuntu also allows you to just install the packages for a different desktop; the differently branded install CDs are mostly for people who come from windows or OSX and thus believe that the desktop == the OS and it's impossible to change once installed :P
