from what I understand;  its because you have a solar system.

if you have just a powerwall;  the grid is allowed to feed it.  If not;  it must only receive feed from the solar system to charge; not from a grid-tied feed.

Similar to the reasoning grid-tied systems can't be connected if the grid end gets disconnected (an outage).  So no back feed can injure a lineman.  It just all has to do with the fact that there are requirements for solar systems that are absolute and can not be changed.  Especially in California.


Now;  nothing is stopping you from re-wiring it yourself;  but... are you qualified?   It's a dangerous combination of systems to mess up even a little.   Hope it puts some light to the situation.   All it takes is the operating of breakers in a completely separate panel, along with the main service disconnect breaker, and a gas generator for a friend to be able to use his grid-tied system when the grid is offline....  Why let 15+Kw go to waste when he's got a business to run.....  But again, im not sure how legal the configuration is; or isn't.  I just know that any modifications are done to his systems while the main service is disconnected completely 110%.