Well I have just one guess. Please consider, I'm still a linux/unix newb, but I guess maybe there exists somewhere the cron.deny-file, where maybe root and so on is stated, or a cron.allow-file, where root and other users are not listed. The idea behind the files is this: If there exists a denyfile, all loginanmes stated there, have no permission to use cron. And if there exists a cron.allow only those who're stated there are able to use the cron.
Now fllows an overview where the ususla files can be found:
crontab-files [USUSAL]: /var/spool/cron/crontabs
crontab-files [RED HAT]: /var/spool/cron/tabs
crontab-files [SuSE]: /etc/crontab/
cron.allow or cron.deny: /etc/ or /var/spool/cron
Hope that helps a little