Usually, it's a more complicated issue, something like: "Historically, grid maintenance costs are amortized over electricity consumption, but when some people become net contributors, that breaks the model, since usage isn't high enough to pay for maintenance."

Of course, the correct solution isn't to compound one perverse incentive (out-of-sync fixed vs variable pricing) with another (ban on solar or refusal to buy); it's to change to a system of decoupled maintenance and power pricing.