It's a gentleman's game. Like in golf, there are expectations of behavior.
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]
> It's a gentleman's game.
Cricket is a game that was designed for toffs to show off their free time, to each other and the plebs (including those making the G&Ts and cucumber sandwiches for the players and spectators) who couldn't take five days out of their lives for a match.
> Like golf
That too.
There is a reason why most other sports income 60/90/ish minute matches: people closer to normal had to squeeze their sports into what little free time they actually had, usually not much more than part of Sunday afternoons or maybe a bit of time some evenings.
> there are expectations of behavior.
While social contracts can be a good thing in terms of helping varied people people get along, cricket and golf are as important in that respect as knowing which of the four forks & three spoons on the table to use next. Etiquette in those forms is just artificial rules by which you show off how "civilised" you think you are, not sportsmanship or other genuine civility.
I suppose my fundamental misunderstanding is that an underarm bowl just seems like the obvious defensive move, not unsportsmanlike.
I said this in another comment and it seems relevant: "I know they're different, but in baseball the pitch is part of the game. Not being able to make good use of a pitch is a problem for the hitter, not the pitcher."
I think my baseballed mind simply cannot warp itself to your gentlemanly ways lol