POSIX is not necessarily brain damaged here. It may appear so from a
narrow Linux-only view though. It probably would have been better if
POSIX actually left this point open, and said the behavior was system
dependent. If there is any POSIX brain damage, it's from being
allowed to delete a directory that a process is using.
But given that POSIX did make a choice, the way they went seems more
system independent to me (not all POSIX systems are UNIX, and ".",
".." might only be logical entities). I sometimes get the impression
that many many people assume standards committees make stuff up
randomly without thinking about them, thus terms like "brain damage"
are used instead of polite disagreement.
It's also important to note that there might be programs that depend
upon the POSIX defined behavior, and they could end up deleting the
wrong things just because someone has declared something as POSIX
brain damage. Traditional UNIX behavior and POSIX behavior are not
suddenly rendered moot just because one particular kernel has no
particular technical reason to follow that behavior.