The MAC framework provides a lot of useful functionality that can be
configured per-jail without requiring the use of labels. Having another
entry point that we invoke just for general prison cleanup rather than
freeing the label is useful to allow a module that can otherwise work
off of a series of MAC entry points + sysctls for configuration to free
its per-jail configuration without having to bring in osd(9).
One such example in the wild is HardenedBSD's secadm, but some of my
own personal use had wanted it as well- it was simply overlooked in the
final version because my first policy made more sense with labels.