There is a difference between “allows hardlinks to directories to exists” and “allows user to straighforwardly create them”. As for existence most unix filesystems have exactly zero issue with that, while on systems that allow you to link(2) to directory you have to be root to do that.
Hardlinks to directories not only break bunch of userspace expectations, but more importantly create a structure that cannot be unlink(2)ed (unlink(2) does not work on directories and rmdir(2) works only for directories with st_nlink==2). So even on systems that allow creation of that it is restricted to root.
Hardlinks to directories not only break bunch of userspace expectations, but more importantly create a structure that cannot be unlink(2)ed (unlink(2) does not work on directories and rmdir(2) works only for directories with st_nlink==2). So even on systems that allow creation of that it is restricted to root.