That means _everyone_ pays the higher price. Water is fungible. You can tax them if you want. But you can’t expect market forces to do it.
I'm not saying that the current rate systems in Mexico, or California, or XYZ place are perfect -- far from it! But there's no reason they couldn't be changed to fit society's goals.
Edit: and they’re still drastically biased. Eg farmers getting preferential rates for wasteful crops like almonds.