I can understand that you might be surprised by the difference in behavior, but you're assuming too much if you think that the UNIX file system style behavior is a "model" that must be conformed to.

It's more of an implementation detail: incarnations of the UNIX file system and many inode-based file systems do things that way. In fact, it is common enough behavior amongst *nix file systems that POSIX/SUSv3 are subtly ambiguous about st_nlink for directories. Yet, if you read one of these standards carefully enough, it is not required for st_nlink for directories to be what you were expecting it to be.