why would they pay for someone else to use their energy? It just eats into their profits and means that their normal customers have to pay more for the solar energy.
I think the power grid should be socialised and electricity handed out for free, especially given our increasing dependency on it. Solar is technically "free" energy given there's nothing to setup and maintain except the infrastructure, versus coal/oil/gas which requires mining, refining and transportation of materials before you get to the generating and delivery parts of the process.
It may be almost free to produce the energy with the infrastructure you have, but it's still limited: there's only so much power available. So how would you allocate it?