This is a common trick used by Marriott and others. They provide a rate in US dollars for your "convenience". Of course the rate is calculated by taking the rate in the local currency and translating it using an unrealistic exchange rate.

You're lucky they didn't try to use dynamic currency conversion to nail you again when you checked out. This is when they bill you in US dollars, again for your "convenience", and again based on a unfavorable exchange rate.