City install water meter to bill for sewage

The park uses well water and connect to city sewer. The city install a water meter to charge for sewage.

Why don’t they install a sewer meter instead?
Anyone encounter the same problem ? If there is a water main leak, the park will be charged more even though those leakage doesn’t drain to their sewage system.

How to deal with the situation like this ?

If your water line has a break usually the City will work with you and reimburse you for what you were charged. I’ve never heard of a sewer meter… if there was one, you’d probably get charged for storm water drainage.