I am pretty sure that everyone bases wastewater cost on water usage. Meters would be difficult to keep clean enough to actually work.

But you are correct that the way winter averaging is used can be problematic. They should use the lesser of the actual usage or the winter average. When I lived in San Marcos they used a system that was particularly bad for renters. Let’s say you move into a rental house in May. They create a new account for you and since you were not there during winter averaging, they use a default value of 5000 gallons. So all summer and fall you are paying for the default usage. Then the winter rolls around and they get your actual usage. And just when your one year lease is up, they apply your actual winter average. If you move at the end of the lease, you start all over with a new account with no winter average