Quoting the manpage of "su" :
PAM is used to set the policy su(1) will use. In particular, by default
only users in the ``wheel'' group can switch to UID 0 (``root''). This
group requirement may be changed by modifying the ``pam_group'' section
of /etc/pam.d/su. See pam_group(8) for details on how to modify this
setting.
so, pam seems reconfigured from stock freebsd.
i do not however see the difference ..