From: Melbourne, Australia
Ok, I think I have found an answer to this. The Rules as Coded, on page 125 under rule 14.4 lists a deviation wrt.
"Deviation: If carrier planes are flying from a land hex, they can only fly air rebase missions."
So, MWIF is behaving as intended by the programmer, even if that differs from the RaW. As to why there is a deviation I cannot say, albeit I think Paul in post #2, gave some insight into the possible reasoning behind that decision.
FWIW, for those interested, I've checked the WiFCE RaW, land based carrier planes can only fly air rebase missions, the same as in MWIF.