I thought that federal law required all electric utilities to purchased it, but they get around it by requiring prohibitively costly interface devices.
There is a large "electric underground" that "sells" electricity back to the utilities on the sly. (They rig up their systems without the interface box, and allow their surplus to spin their meters backwards.) You should be able to find some info via google.
IOW, I do have a bidirectional meter, it is calibrated to spin backwards when I'm generating, but if the end of the month balance is negative the "surplus" power I generated is a gift to PG&E.