nivi, they have to destroy every fleet in the sea in order to do a naval landing attack (onto territory they dont own, like last turn). but when its to their own lands, your fleets wouldnt automaticaly defend it.
think of it as a timing thing. by the time your 13 fleets from C arrive, they've gotten the armies across already.
you could argue that the 5 you built were there before he would have gotten accross, but if those 5 fought the 17 he had, they'd have sunk for no reason (maybe taking 2 or 3 with them) and you not only would not have stopped the transport, but lost the major battle afterwards. (odds wise that is)