They meter the sewage by the amount of water you use, so using an un-metered water source would short-change them. You do have the option of getting a separate water meter for water you use for gardening, to avoid the sewage bill for water you don't send down the drain.
I've never heard of metering sewage before. Everywhere I've ever lived had a flat fee per home, or based it on square footage. Is the water utility owned by the local government?
Sewer billing is a thing where I am (Cincinnati, Ohio), and I can tell you exactly why!
The sewer system in the urban area was largely built out by about 100 years ago as a combined system, where rain water and sewage were in the same sewer pipes. Many people didn't really care about water quality and disease back then, so the fact that the Ohio river was a big stream of shit and chemicals didn't matter.
As other parts of life improved eventually more and more people wanted clean, healthy water and surrounding ecosystems, hence the EPA was created 50 years ago. The EPA got stricter and stricter over the last 50 years, and about 20-30 years ago started cracking down on sewage outflows in to waterways.
As a result, the local waste water treatment utility (owned by local government) has been under a legal decree to fix sewage overflows which occur during storms due to these combined systems. This has meant spending billions on tearing out these pipes and replacing them with separate storm water and sewage lines.
The utility started charging for separate clean water usage and sewer usage so people understood a bit better why their bill went up so much.
I'm curious about why that's not allowed