I am sure that was a loophole in the rules - an unintended exploit, for what it’s worth.
Nevertheless, it does cost the Allies to do that tactic, and if the Axis player knows about it he can keep from having all his transports in one zone, or just leave them loaded the turn before.
It costs the Allies - they have to do an unprovoked DOW, UK can’t do it so they lose out on the 5 from the NO, and it costs UK a destroyer too.
Also, you are doing an unprovoked attack but the UK can’t declare war, so can’t sucker punch Japan anywhere. Only ANZAC can.
I do realize that to leave some units on transports decreases Japan’s threat level on Yunnan and the like. But it’s not such a great exploit that it would even happen more often than not. Something to be aware of as Axis, though.