In many states, power-production has been decoupled from power-delivery. For example, in Illinois[0], customers can choose which company to buy their power from but maintaining the lines and delivering that power is still the responsibility of Commonwealth Edison, the old monopoly utility. This seems like a reasonable compromise to me, and it's not that dissimilar from Comcast delivering my packets, while I choose to buy my content from Netflix or Hulu or Amazon.

[0] http://www.pluginillinois.org/default.aspx