But even then there are other problems: the OS will have to either 
 :a) be able to automatically delete entire directory tree with recursion (i.e. rmdir doesn't require the dir to be empty or have more than 1 link) or 
 :b) disallow rmdir for problematic cases, which can very unpredictable and unexpected for the applications (e.g. in the above case allow "rmdir /B/A" but disallow "rmdir /A").