In my early years of visiting with friends, back when it wasn't as easy to use island ATMs and tap/charge everything, we used dollars since we were usually only there for 5 nights and it was easier with a group. We always asked for bills to be converted to dollars and knew we were paying a higher exchange rate but, as Ellen said, restaurants provide a service in doing so and thus charge a higher rate. For us it was a convenience and the difference, when divided by 6 or 8 people, usually didn't add up too much. I think we even used dollars at the old Marche U (!) the cashiers would calculate, bless their hearts. That was 20+ years ago but the fact remains, as crazy as it sounds to those of us who visit regularly, many Americans still come with dollars and use dollars. I think more and more restaurants provide the conversion on bills as it likely saves an extra step for the server when the customer wants to pay in dollars. I'm not sure first time visitors (or others) necessarily know they are paying a higher rate in dollars but as far as I know, it's always been this way.
My earlier mention of a charge from Guanahani last year felt sneaky because my card was automatically charged in dollars (at a higher rate) without my knowledge when I expected to be charged in Euros...that might be considered a scam as the hotel was charging a much higher rate, thus making extra money.