From an tactic point of view this rule is better, if we consider it together with the new transport status.
Surface fleets and submarines did not operate togheter during ww2. So having the sub (and the TRNs) as cannon fodder for surface battle is not historically correct, is not tactically correct and even from a gaming point of view it hampered war on the seas.
In this game the real backbone of the fleet is the DD, that should be built in good numbers.
It is true that a fleet with AC and submarines as cannon fodder could suffer grievous losses to air power… but also a fleet with a lone DD if attacked by a bunch of subs (that cost 6 i.e 4 IPC less than a fighter) could be wiped away leaving fighters (immune to subs fire…) to look for a place for landing.
So I agree that it is possible to use air power for trading with capital ships (right what happens in the real war, the aircraft pilots are trained to attack valuable target not useless TRN or SUBS… imagine the battle of Midway and USA Dauntless pilots sunking four submarine instead of four AC…) but also subs may be used for trading with any kind of target that is not an aircraft, i.e. DD, CA, AC and BB. I see a paper-scissors-stone pattern in that.