Excess energy on the grid is a real issue, especially if there has been a significant wave of people adopting these systems. If there isn't demand for all the electricity being pumped onto the grid, there has to be a place to dump the energy. This is an even bigger issue with wind and other intermittent sources.
If the grid is overwhelmed and there is no demand, should folks expect to get paid for that energy, which could actually cost the utility money to dump somewhere?
Something else to bear in mind- the utility has to operate base load plants no matter what.
Recent literature indicates that these issues can be overcome (one example from Utilities Policy ), but that the process will take time. Utilities are a very conservative industry and are often slow to adapt new systems because they have stringent boundary conditions.
Just playing the devils advocate here- I'm sure profit is a part of it.