well, my reasoning was that, between the efforts of the Athenians and the Cypriots(sp), Persia now has a fairly good open-water fleet (note that they control Knossos, and possibly dominate trade with Egypt)...which makes them a threat to Carthage.
btw, I never said the Punii mothercities are loyal...only that they're part of Persia.
OTL Carthage also had a treaty with Rome as well.