Puerto Rico does use USD. But in case you haven't noticed, most (all?) credit cards that charge forex these days do so for "foreign transactions" (ie, they don't care that the transaction is in US Dollars, they just care that it was made outside of the 50+DC USA), not just for "foreign currency" (as they were years ago).

So while they do use USD, the credit card companies (dunno if all, but enough of them) define it as "foreign" for the purpose of classifying what is a "foreign transaction".

I would presume that forex is also charged by those same credit card companies for purchase in Guam and Saipan.

Now, having said that, the Marriott Premier Visa does not charge any forex at all (no foreign transaction fee, no foreign currency fee), so it is in fact my go-to card for foreign travel. It's just that I also apparently have to make it my go-to card for Puerto Rico travel (despite most of my stays in Puerto Rico being with hotels in other programs!).