There is a bug here, but your assertion that it should interpret "start" as "unmount" is incorrect -- from sysv-rc README.runlevels (same applies for rc0.d):
"Then the /etc/rc6.d/SXXxxxx scripts will be executed alphabetically with "stop" as the first argument as well. The reason is that there is nothing to start anymore at this point - all scripts that are run are meant to bring the system down."
There is a bug here, but your assertion that it should interpret "start" as "unmount" is incorrect -- from sysv-rc README.runlevels (same applies for rc0.d):
"Then the /etc/rc6.d/SXXxxxx scripts will be executed alphabetically with "stop" as the first argument as well. The reason is that there is nothing to start anymore at this point - all scripts that are run are meant to bring the system down."