Would you allow the TAB to accept all in bets on scratchings becareful?
By your logic that would be ok and you would have the same chance of winning as with these ridiculous 3YO quotes. If someone is stupid enough to bet on a scratching let them do their dough.
The problem with these doubles is that the less switched on punter may not be aware of Melbourne Cup qualification rules for 3YO's and think a Davidoff-Platinum Scissors double for example is a chance. It has no chance as it cannot happen and the TAB is reaching a new level of crookedness allowing unsuspecting people invest money with zero prospect of a return.
I bet one of those evil bookmakers wouldn't give you a quote on something like this - most of them now understand the importance of client satisfaction; a concept lost on the TAB.