This page dates back to the early 2000s, when Apple was a relatively small company after years of decline, and OS X was gaining popularity among UNIX users. They hired some people from the FreeBSD project (most notably Jordan Hubbard, one of the founders of FreeBSD). This was also the time that everyone and their dog was going open source (a trend started by Netscape).
The industry's open sourcing spree, their traction in the UNIX community, and the fact that they were importing a lot of new BSD code, probably moved them to make non-Aqua/Cocoa source open source.
Suddenly removing source code now would probably be a small PR disaster. So, they basically let stuff rot: some new stuff is missing and they are slow in posting new code drops.