Looks very similar to the requirements as specified in the T&C when booking a Thrifty reservation in Costa Rica.

Basically, they have you either way: you are required to take CDW. But if you can prove that you have CDW coverage from something else (typically your credit card), you don't have to take the CDW. In that case, though, you'll be required to take the ALI. If you decline the ALI, you're required to take the CDW.

So, either way, you're required to take either CDW or ALI, and there is no way around that (short of some of the all-inclusive rates mentioned above).