Because if sys-apps/shadow is built with USE=audit, even sys-apps/portage will not work anymore after an upgrade to >sys-process/audit-2.
This bug is critical, if you don't re-emerge pam and shadow before a reboot or a disconnect you cannot login and so must use a livecd to repair the system.
Also, revdep-rebuild starts emitting lots of scary (but harmless) warnings.
InCVS.