If I want to support Windows I have to setup a (virtual) machine with a license code just to have any basic functionality. As someone who doesn't work with Windows anywhere, this is a huge hurdle. Then there's the hoops I'd have to jump through just to get a decent shell and a compiler ready.
Oh, but the fun doesn't end there. Which version of Windows are we talking about? More than one? Ah, well, each of these will require its own machine (virtual or otherwise). With matching keys, of course.
Supporting Windows is a pain and when it's not required for what I want to do, I'm simply not going to do it. Period. I have better things to do with my time.
Letting people who want this code running on Windows do it themselves and having them submit patches upstream is a perfectly valid choice.
It's not like going from Unix to Windows, where you have an entirely different access control mechanism, system API, filesystem structure and path separator, etc.; it's just going from one Unix to another. So the author only has to accept a couple patches from his Mac-using colleagues, and then he's basically set for OS X. On the other hand, debugging a Unix app on Windows can become a full-time job, and may be more work than you can reasonably expect a few random patch contributors to accomplish.
As for newbie devs, I'm not sure I agree... as a non-newbie, I still like to get a solution that works. I might be able to dive into the code and fix it, but I don't want to do this - I prefer to focus on my app rather than figure out why certain libraries don't work.
(For some reason this view seems to be common in Ruby. A prominent example was the web server - until Passenger came along (surprising everyone) you were supposed to figure out how to deploy with lighttpd, then mongrel - instead of simply working with Apache which most of the world already knew how to install, scale and tune.)
As far as deployment goes, when you can run Windows services for the mongrels, do you even need capistrano? I've been doing Rails for years without it and don't feel like I need help deploying....
I think this issue, like Capistrano's, would benefit from having more Ruby coders, which in itself might require a more newbie-friendly approach from the community.
(BTW I hope it doesn't seem like I'm criticizing Jamis' choice - actually I think he's doing the right thing in the current situation, just that if there were more Ruby coders around he might not be forced into choosing this path)