Nebular
Chieftain
Your basically arguing that it is stupid of water to run to the see, if it can be used to irrigate a desert. The water couldn't care less. Usefulness does not enter into the equation for cultural expansion (or only as a small bias), the culture simply goes where it is easiest to go, like water.
By that logic then, it should always expand to the tile that offers the fewest resources which is not always the case (and would frequently result in cities with only a handful of workable tiles and a bunch outside of that 3-tile radius). I've had a number of times where the first tile a city expands to is a high-yield tile. Things like sheep generally don't resist culture anyways.
