I'm guessing, as previously stated, that they're lightboxes. Like, they're placed at all sorts of angels and degrees under SW. It's probably to remind the devs what the light angles look like on buildings.
That, and it can always be skyboxes for buildings? 'Cause seeing your minimap changes when you go in a building, and that if you'd ever been on a private server, there looks like empty space when you're outside the building but not outside of it.
EDIT: Here's the direct link to the WoWMapViewer. It's a bit old and buggy, but still serves its purpose - http://wowmodelviewer.googlecode.com...alfred-r90.zip