Uh, because their timelines go backwards. Think about it, everything happens backwards. So every time River has met Rory in the future, she knows who he is in the present.

But for that to be true, Moffat would have to go back and revisit every episode.

I think it's actually an interesting hole that River knows Amy and Rory, when theoretically she should have to be introduced every time, in a manner of speaking.