Honestly, I'd've happily taken "because we want to remind people of the Mac, not win32" as a reason. Which is what I inferred and seemed perfectly good justification for a "we said so" type decision.

Of course, in light of this I may be blowing smoke out my ass here, but I'm still in favour of it for that reason even if it wasn't Canonical's reason.

"Our intent is to encourage innovation, discussion, and design with the right of the window title bar. We have some ideas, and others are already springing up in the community."

https://bugs.edge.launchpad.net/ubuntu/+source/light-themes/...

I think it was mentioned in other places too but I can't find it now.