Tiens, extrait du man crontab :
Unlike other crond/crontabs, this crontab does not try to do everything under the sun. Frankly, a shell script is much more able to manipulate the environment then cron and I see no particular reason to use the user's shell (from his password entry) to run cron commands when this requires special casing of non-user crontabs, such as those for UUCP. When a crontab command is run, this crontab runs it with /bin/sh and sets up only three environment variables: USER, HOME, and SHELL.