That is exactly my point, though I could have stated it more clearly. A good history is always more complicated that a simple friend/enemy switch.
If we take, for example, the American civil war, we can see that there were many underlying factors that influenced people's views on the matter. In the end, it led to war, but families were on both sides, there was the slavery issue, and with the European connections, it was all a giant mess.
So if we take the "gods cursed the landdwellers" idea for a moment, it wouldn't provide a clear answer so much as a background for history, an environment for your npc's to fall back on. In different areas, this would be dealt with in different ways. Voila, instant cultural differences, with a reason.
I am sorry if I preach too much. I have to stop myself from lecturing sometimes. :P