The laws of supply and demand are skewed here because the Aiel could cross over the Dragonwall at any point to lead lives next to water.

It is their belief system and lifestyle that create the value they live their lives by.

You can flip it on it's head and say "If water is so valuable to you, why do you choose to live in a place where you kill each other over access to it.", but the response is going to "Because we are Aiel." i.e. based on an illogical assertion about cultural identity.

This is keeping in mind that the Aiel contain some of the few characters that understand Lan's point of view & sense of humor and vice versa, so it isn't shocking that they both have belief systems that correspond to each other in their fatalistic irrationality.