Originally Posted by
Snick
In this case, is Puerto Rico a different country in Amex's eyes?
In many cases, Puerto Rico is a different country in various banks' eyes. In that you're likely to be charged a foreign transaction fee when you use a card with such a fee for US dollars purchases in Puerto Rico.
So given that, I would think that it's very unlikely that restaurants there (or in Guam) will be counted as "US restaurants".