based on the story above I agree, knowingly (and after a few occurrences you should understand there's a problem) damaging the service should lead to a ban or at least a warning.
One facet of the problem (the cell coverage) is largely out of Zipcars control, no? I suppose they could add some functionality to their app so that you're alerted when you park at a location without cell coverage?
Lack of cell coverage is out of their control, but that doesn't excuse them from designing around the problem.
It appears they're aware of the problem, but certain known issues prevent access (cellular unit draining car battery seems to be the common one and sounds like something that should be fixable).
Add a keypad like Ford has had on their vehicles for like 20 years. Have the computer keep a short list of one time pass codes (and maybe some sort of super long master code) that can be updated whenever the car is in cell service. So in the event that you get into the car, Zipcar can provide one of the one time use codes to get you in the door.
My imagined internal security review to this issue Hard "no", closing WONTFIX. This change would significantly increase the attack surface, risking a lot for very little upside. See "Epic 769 (Offline Support - roadmap Q3 2020)" for blocking & related issues