$ls -ia 2 . 2 .. 3963 bin ...............
3.144 Empty Directory
A directory that contains, at most, directory entries for dot anddot-dot, and has exactly one link to it (other than its own dot entry, if one exists), in dot-dot. This definition is repeated with different phrasings in the POSIX descriptions of all file-commands.
This above convoluted definition for an Empty Directory is apparently behind this funny convention, and its purpose is to avoid any exceptions to the rule, not even for slash (/) i.e. root dir.