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.
As i understand it, there are folks in Australia (and elsewhere in the US) who are doing exactly what I was told I could do by Tesla salesfolks (fill up on grid overnight, and sell solar generated power to PG&E) - but because of PGE, there is some rule in place which prevents the storage (w/solar) from filling up from the grid. This is the precise detail which basically rekt my ROI on this system, and gave me a storage system which is near worthless on consecutive cloudy days.
The Telsa forums are full of folks trying to circumvent this silliness (by feeding other power threw the Tesla transducer / current sensors at night, 'tricking' the Powerwall sw to think there is solar being generated) as well as other methods, but your mention of re-wiring it definitely piques my interest. I'd love to have that done, if it's actually feasible - can you elaborate at all on that aspect?