I tracked it down to 6628, which I believe was an attempt to mimic KoL's familiar locking, except without removing specific equipment that a familiar might already be wearing when you switch to it (a reason to avoid KoL's familiar locking). From the perspective of doing something similar to familiar locking, it makes sense to assume nothing is "locked" if nothing is equipped. If there was any discussion about that decision, it was lost when the old bug report system went away.
A way to choose whether to put on equipment would be simple to add, if it's actually wanted. More useful might be a way to customize the order of things listed, but I can't even picture a good GUI for configuring that.