Based on your financial expertise, provide your response or viewpoint on the given financial question or topic. The response format is open.

Which credit card is friendliest to merchants?
Merchants that accept American Express should have decided that the extra costs are worth the increased business (many business travelers only have an Amex Corporate Card).  To complain about people actually using it after they've explicitly decided to accept it is a sign that they made the wrong decision, or that they are very short-sighted.  No one is forcing them to take a particular card.