As well they should set their own rate. The actual legal tender in the French side is the euro. Businesses are, to some extent, doing you a favour by accepting any other currency. Addiionally the business does not get the rate that is posted. There is one rate to buy fx and another to sell it. Their banks charge them a service fee just like some credit cards charge a fee. The business shouldn't have to eat that cost for doiong you the favour of accepting your currency.