The reason is that we don't want people starting to track other people's movements. Although it's not a big deal to some people, it's still quite a big deal to others. There may be no compelling need to close it off to everyone except the administrators, there is no compelling need to open it up either. Coupled with my second point below, we've decided to close it off to everyone except administrators only.
Opening it up to everyone also adds additional load to the server, which results in the server slowing down.