https://www.dmv.ca.gov/portal/file/adopted-regulatory-text-p...
I didn't see any language demanding that the vehicle become inoperable in a cellular dead zone, but if their paramount concern is to shield themselves from liability, then having the car disengage is one way to ensure that.
For residents of San Francisco that were personally affected by this, consider writing a polite note to the DMV asking them to ensure that this incident is included in this winter's upcoming disengagement reports (227.50) from Cruise, and share your point of views — both on how you expected autonomous vehicles to behave when cellular communication is disrupted, and on whether you think that the existing guidelines are sufficiently clear about that scenario. <autonomousvehicles@dmv.ca.gov>