They're in the business of making money for themselves, not allowing you to make money at their expensive.
I mean you really should be happy that they allow you sell back power in general at a 1:1 rate, they fought very hard to try and reduce that, in fact there are places where they don't. Bottom line they're the energy monopoly, they don't want the little guy breaking into their gig, even though small areas could easily be PG&E free and not part of the grid as a whole with enough solar and storage solutions (Powerwalls or other brand batteries).