I've looked into the insurance thing a bit more, and I think there is more to the story.
1. It is Mexican law that cars must have 3rd party liability insurance; no car rental company can let you drive the car off the lot without having it.
2. The Avis form says "ADDITIONAL" personal liability insurance. To me, that means that the basic coverage is still in place, and is included in the rate.
3. If you don't purchase the additional liability insurance and proceed with the reservation, it shows that you declined "additional" liability.
I believe the Avis policy is the same as it has always been: the basic 3rd party liability is included in the rate quoted. If you aren't happy with the amount, they offer additional liability insurance.
I absolutely know they can't allow you do drive the car without liability insurance, and since they will allow you to rent without taking additional liability insurance, there is liability insurance included.........I THINK. Before renting again, I'll find out for sure.