Well, cooking and drinking water end up in the sewer at some point, unless you have an outhouse or composting toilet. In which case, you should look into tax credits and/or utility exemptions.
With gardening water, of course, you have point, but there are really only 2-3 months of the year when you need to water a garden here, and if paying the sewage charges for that water really chaps your hide, it's not all that expensive to implement a rainwater collection system, and then your garden will be "off the grid".
The water rate calculation is an approximation that's reasonably accurate for most customers, and probably more cost effective for the utility and the rate payer than implementing equipment to measure each individual's actual sewer usage. And sewage treatment is an absolute necessity in a city. With 608,000 people, you can't just empty everyone's toilet directly into the Sound and the lakes.
What do you think would be a better way to calculate the charges?