It might also have to do something with demand and income levels. As both of them rise people who could afford buying unadulterated milk meant the vendor had bi

It might also have to do something with demand and income levels. As both of them rise people who could afford buying unadulterated milk meant the vendor had bigger demands to meet. Now he could either raise the price, something which might be frowned upon. Or start diluting the milk unless he is caught and forced to raise the price.