> Well they sort of do already — typically you pay less for water for irrigation (usually because sewerage is metered by water usage and obv if it goes on the lawn it’s not going down the drain).

Aren't you paying more for your lawn water then?

If your sewer bill is based on your water bill[1]: a gallon of water for drinking results in a bill for a gallon of sewage treatment, which you'll use. A gallon of water for irrigation results in a bill for a gallon of sewage treatment which you won't use. Caveats: maybe you pee on the lawn, probably you perspire, sewer pipes are leaky: some of your sewage escapes out, some of your irrigation water escapes in.

[1] this is common, but I don't think anywhere close to universal; even ignoring lack of universal municipal water and lack of universal municipal sewage. Flatrate by connection size is also common. Approximately zero households have individual sewer meters, but some commercial/industrial customers may have them so they can be billed on actual usage.