This extra rule, that adds nothing, causes confusion with some players.
But it really doesnt add anything to the game any reasonable intelligent person would not think of themself.
What happens if you remove the rule, well since only the attacker can roll dice he will roll dice untill the transports are destroyed. Any sane person would see this as inevitable so they would just take away the transports and save time.
So there is no reason to even have this rule in the rulebook. People read half the rules sometimes, start thinking about scenarios that dont apply and then come up with questions.
Can solo transports retreat when they are attacking, yes.
Can i bombard if i only fight transports, NO,
but they are defenseless there was no sea battle, there was a sea battle you just optimised it.