I'm not sure that I agree.
I think that a story has to be fluid and make sense as far as plot line goes. I think there has to be an explanation for everything that happens. But as long as there is an explanation for what it happening, why can't something not make sense?
Aliens are fiction and they don't really make much sense, but that's fully within an author's right, isn't it? Why couldn't someone write a story about a group of Amish friends fighting a little known battle in World War II? I mean, that really doesn't make sense, but why isn't that within an author's right as a work of fiction?
I always took fiction to mean that an author can make whatever he/she wants, so long as it makes sense in the context of the fictional world.
If that is incorrect, what is fiction?
EDIT: I suppose I read the question in a different context. Thanks, Lyra! Originally, I thought this question implied that a fiction story had to make sense in the world we live in, but it's talking more about breaking a world's rules, no matter what the world is and justifying it with "it's fiction!"? I can understand that. And I would agree that you really can't do that. There are always rules you have to follow.