There is no mention of liability insurance at all, even when booking directly at Enterprise.com in the US (we checked). CDW is listed as an option as are other protection products:
As you can see, it's not possible to add liability insurance via Enterprise's own website. Furthermore,
according to AutoEurope:
When renting a car in France it is required by law that drivers carry unlimited third party liability insurance, and as a result, it is automatically included in the price of all car rentals. The advantage of having this car rental insurance in France is that you are protected from injury, death or property damage to others which occur while operating your rental during your trip to France.
At best, you were given incorrect information inadvertently, but given your explanation of the circumstances, it seems that the Enterprise reps intentionally misled you into abandoning your Costco reservation and forced you to take the extra insurance from them. What you probably bought was a higher level of liability coverage above and beyond what's normally included, that was bundled along with CDW. It's hard to say for sure without looking at your actual rental contract. It bears mentioning that by forcing you to do this, the Enterprise rep may have received a commission for an add-on sale, and without a doubt, Enterprise weaseled out of having to pay Costco a commission for the rental (yes, Costco makes a commission for every rental booked through CostcoTravel.com).