They didn't think they needed a rule.
This was what made me certain they were wrong--the commentary of their own older brother, who's hugely respected:
> As the ball was being bowled, Ian Chappell (elder brother of Greg and Trevor, and a former Australian captain), who was commentating on the match, was heard to call out "No, Greg, no, you can't do that"[10] in an instinctive reaction to the incident, and he remained critical in a later newspaper article on the incident.[11]