People who install solar on their roofs usually expect to recoup some of the costs by sending energy to the grid. When, increasingly often, they have a choice of either shutting the system off and wasting this energy or sending it to the grid at low or even negative rates, this becomes a problem. The expectation of "my solar system will pay for itself in X years" might become "my solar system will never break even". At least that's an issue in some places with high PV density.