Once per phase could lead to the following:
A player moves a lone unloaded transport through a sea zone containing 5 enemy subs. The subs attack, most likely destroying the transport. That player now safely moves 5 fully loaded unaccompanied transports through the sea zone.
Of course, this may be the last game of A&A that player plays with that group, but still.
You could choose to not shoot at that first transport, obviously the player is trying to be sneaky, best wait for the juicier target.
What if the player tries to move them one at a time through the sea zone? as a way to try and insure only one can be destroyed, or as a way to make it so only one sub shoots at each transport. Lets say the player has 5 transports that might make a move through the sea zone containing 5 subs. He moves one. OK, he’s up to something, I’ll fire with just one sub. He moves another, I’ll fire with another sub. He moves a third, I’ll fire with my third sub. He doesn’t try to move any others through the sea zone. If he had moved them all at once, all 5 of the subs would have fired, making a kill more likely.
I don’t really expect the errata to try and cover this situation, though. I can take care of it myself, with a quick toss out the door.