In many countries, governments set standards for domestic water. The companies selling the water must clean the water of pollutants in order to meet these standards. Polluters are not charged for fouling the water, leaving the individual end user to foot the bill for water cleansing.
Farmers take advice from pesticide companies on how much pesticide to use, and of course the pesticide companies have a vested interest in selling pesticides. Moreover, the British government has abandoned its principle of making the polluter pay, exacerbating the both the pollution and the unfair billing of the consumer.