It's not there for any particular reason I'd say, would just make sense from a more straight to the point matter.
I believe when razmirz put it there, I believe it was made just because it's easier to make it so all map processes on the server show, rather to code something up that detects "This is a private map, therefore it shall not be listed". It would give it more work than it was worth I believe, so to answer your question, it's probably due to simplicity of making it so, and personally I find it more handy to have all maps with most users show rather than only public ones.
Why work more for less?
Is probably the best way to put it.