For the record, same for PayPal. Not a major issue I suppose but when paying for USD purchases, I've been offered PayPal's fixed rate at the time of payment ('purchasing with confidence'). I've noted the amount it would have cost me, deselected it and allowed my credit card company to set the rate. Each time it has come in roughly 1% less (in Euro terms) than the rate I had been offered.
Granted credit card FX rates aren't majorly competitive but it would seem that offering someone 'confidence' in their purchases comes at a cost!