That is a quirk: cron uses a log function in busybox that has no input for error levels and is fixed to log everything as an "error." Consequently every log entry by cron will be tagged cron.err, whether it is an actual error or not.
What you can do, however, is set cron itself to a different error level, it will internally decide what to send to the log -- see the cronloglevel option in /etc/config/system. Personally I set it to 9, so it will really only send actual errors.