They call it a safety feature.. lol.. So here's how it works.. the solar panels produce energy and it goes to an inverter that converts it from dc to ac.  That inverter only stays powered on and converting energy as long as it sees power coming from the grid.  As soon as the grid stops producing power, the inverter shuts off and stops converting dc to ac.  The claim is it prevents someone working on the down lines from being electrocuted.. They could simply add an automatic shut off that disconnects the main from your house when the grid goes down.. but this adds cost so its not done.  You can also get a back up battery and use that to feed ac back into the inverter once the main is shut off.. This is a pretty simple solution and easy to do to keep your solar panels producing power without the grid being up.