I had the same sentiment as you until a few days ago... and H2ofun's thread about grid-charging the Powerwalls revealed that PG&E does seem to encourage using batteries to "get the most" out of a system.
Battery storage for your home
I mean, maybe this is being taken out of context, but they say:
Potentially reduce your energy costs: If you are on a PG&E Time-of-Use rate or Home Charging rate, your battery can charge when electricity is cheaper and discharge for use in your home when electricity from the grid is more expensive.
Since most people on TOU rates already have solar or EV's, it seems PG&E actually wants people to add ESS to further take advantage of TOU. They don't seem to be against grid-charging when rates are low in order to then use that energy when rates go up.
So yeah, the only thing stopping a grid-charged-Tesla-Powerwall is their adherence to the federal ITC requirements where batteries only qualify if they're not routinely grid-charged.
I'm very interested to see what H2ofun found in terms of there being a 5 year expiry on this restriction. It seems that eventually (maybe) Tesla will allow a ITC-funded-ESS-with-solar to grid charge at night to allow almost a reverse TOU to bank solar at higher rates than nighttime production.
But with the math I've estimated, a cloudy day is going to cost a boatload... I won't be able to fill the Powerwalls with enough self-generation to cover sunset-to-midnight, so I'll need to take energy from PG&E at the worst possible rate.
If I could grid-charge on command, then I could monitor weather forecasts and max out the Powerwalls overnight to enter a cloudy day with a bank of energy to get to midnight without any behavior modification.