It’s not both ways. The customer understands that until Enterprise comes into possession of the vehicle, it is her responsibility. If a customer signs a contract to that effect, should that signature be considered meaningless? I am truly sorry for the customer that this happened, if I haven’t said so.
You are mistaken in thinking that Enterprise will pay for the Mustang. Every other Enterprise renter will pay for it, via increased costs.
And yes, Enterprise customers will pay, at least those who remain. But Enterprise itself is also going to pay for all this bad publicity. Bad move on their part.
It is both ways if you want to penalize someone for not returning something on a given date but also hold them responsible for other potential damages if you do return it on that date.
Hence both ways.
If Enterprise requires something returned on a particular date they better be prepared to be responsible for it on that particular date. My bet is a court will see it the same way.