People who have solar, who sell solar, who install solar, want to install solar, or who think distributed solar are a public good worthy of subsidy want to keep this rule.
In addition, the price of energy and the economy behind new plants is based on the commercial peak ("please run your washing machine in the afternoon", used to remember you state-run companies with every bill). Nice graph at http://www.mpoweruk.com/images/elec_load_demand.gif from http://www.mpoweruk.com/electricity_demand.html (please note that that graph is from 1999's Californa; now the commercial spike is much higher due to air conditioners being active in almost all office).
Thus, solar helps when it is "needed" the most.
I.e., using the home's temperature as a very inefficient battery but better than throwing it away.
So as people start generating their own power, transmissions charges go down. This means that transmission charges for people not generating their own power go up to cover the mostly fixed cost of maintaining the electric grid. I think charging a reasonable transmission fee (or toll) per kWh sold back to the power company is a reasonable compromise that prevents people from benefiting from a fully maintained electrical grid, but not paying for it.
[1]Obviously it's impossible to tell where any specific user's generation came from