Setuid para diretórios não se comporta como setgid. A menos que a saída do shell fosse do FreeBSD, alguém estava entediado e se divertindo um pouco às suas custas.
The setuid permission set on a directory is ignored on UNIX and Linux systems.[4] FreeBSD can be configured to interpret it analogously to setgid, namely, to force all files and sub-directories to be owned by the top directory owner.[5]
In FreeBSD, directories behave as if their setgid bit was always set, regardless of its actual value. As is stated in open(2), "When a new file is created it is given the group of the directory which contains it."