Since their net use is zero, their electric bill is zero
Wrong for several reasons.
Some households that participate in net metering are net producers of power, "exporting" more than they "import." Other households have remained net consumers of power. But no household has perfectly balanced exports with imports, resulting in zero net usage.
And even if a household did happen, one month, to export exactly the same number of kilowatt-hours as it imports, the fact of net metering does not guarantee that the utility will pay retail price for the exported power. The term "net metering" also applies when a utility pays wholesale price:
Net metering policies can vary significantly by country and by state or province: if net metering is available, if and how long you can keep your banked credits, and how much the credits are worth (retail/wholesale).
It doesn't make sense that a utility should be forced to pay retail price for each tiny trickle of power generated by amateur mom-and-pop producers, when bulk power generated by professionally-managed plants can be purchased at wholesale price.