Easy: crontabs are in /var/spool/cron/..., not /home, and user editable. If the user's homedir isn't mounted because they logged off and cleared their security

Easy: crontabs are in /var/spool/cron/..., not /home, and user editable. If the user's homedir isn't mounted because they logged off and cleared their security tokens, any crontab they write that uses their homedir will break.