@gamergray:
What prevents the SS from engaging the AP(transport)?
The sub stops the transport if is is alone.
If the BB or CA don’t engage the SS why does the SS not stop the transport?
Now I understand your question.
Subs can be ignored.
An exception to the rule is unescorted transports (sub prevents that combat movement from being made)
However, in this case the transport IS escorted, therefore the exception does not apply and you are back to subs can be ignored.
However, if there is a scramble, then there is combat in the seazone and the sub joins in the combat and cannot be ignored (after all, it is not alone - it has defending aircraft with it).
In other words, a warship can always ignore subs unless there is a scramble of aircraft, regardless of whether transports are with it/them or not.