The reward had a caveat about the offense (of covering up/knowing, I think) being negated if a person told where William is.
"Inspector Jubelin said rewards were usually paid out on the arrest and conviction of the offender, but the recovery of William had been added as a condition on this reward.
If you've got information regarding William's disappearance and you've got concerns that you could be potentially complicit in the offence, by concealing the offence, that offence will be negated the moment you come to police," he said.
"That's a big ticket item that you can consider, if you have been sitting on this information."
$1 million reward offered in William Tyrrell case