What are bidirectional Zener diodes used for in DC transient protection?
Automotive Design standards are very strict. 

Electronics must survive a car being jump started in reverse and boosted with 24V. (Yes people still do that in dead of winter) It must also handle load dump with a battery under load being disconnected.

If your design shunts the battery at -0.7V it will blow like a fuse then you have no protection.

So be warned automotive tests are much stricter.
This particular design of zener is to protect the regulator from failure within its abs. max input. of -40V ~+80V, which might seem enough but these zeners will not blow in the automotive +24/12V test yet protect the device from additional spikes above that.