lots of places all over the world accept dollars, the registers are programmed to figure out the amount required and the change (usually in local currency)
2 points-
there is usually a rather high % figured in, stores are normally the worst place to exchange, so they are making more $$$ off the customers to offer the convienence.
think of it this way, it means that tourists and business people are spending money IN our country, so maybe it's not as bad as it seems.
and, all other things being equal, maybe they are selling more by offering the service.