Why should we subsidise the dairy industry?

The Green-Labour policy to make commercial water users pay for their use of a public resource has farmers running scared and whining that it will force them out of business. To which the response is "so?" To point out the obvious, that water is a public resource, and it is only fair that farmers pay for it. Letting them not pay for it is effectively subsidising them. And why should urban New Zealand subsidise environmental destruction for the private profit of a few?

The same argument applies to the whining about Canterbury dairy farmers going out of business if they have to clean up their act to save Lake Ellesmere. These farmers are currently destroying a public resource, causing (according to ECan) $300 million in economic damage to it per year. Which means that we are effectively subsidising them by that sum. And why the hell should we do that?

Polluters should pay the full social costs of the resources they use and the environmental damage they cause. If paying those costs means that they are no longer (privately) profitable, then they were never profitable in the first place - they were simply hiding that fact by getting us to pay for their losses. And in such situations, the best thing that can happen to such businesses is for them to go out of business as quickly as possible, so they don't cost us any more money.