I actually asked a vegetarian friend this once. She said the reason she was vegi was because she disagree with how the farming industry works, if she were able to keep her own animals and make sure they were properly cared for then she would eat meat. So fish (assuming no from a fish farm, but out in open sea) are ok to eat because they aren't subject to conditions.
I don't know whether I agree with that or not, but that was her logic.
As for vegis that don't eat fish because "eating animals is inhumane" - well I have no idea.