It doesn't seem to be true for other mount points; the ones I tried raise FileExistsError.  I suppose one could dig into the OS sources.  I see that none of the FreeBSD or OS X mkdir man pages nor the POSIX 2013 spec document EISDIR as an expected error from mkdir(2) so one could argue it's an implementation bug.  It is a somewhat unusual case, though, as I doubt you would ever run into a situation where you really needed to create '/' from a process :=)  Still ...