AMEX could negotiate their transaction fees down, right? Walmart is very well known for playing hardball with suppliers, so maybe they give credit card companies "take it or leave it" offers as well.
You bet that Walmart negotiated an excellent rate with Amex.
[1] http://www.moneysense.ca/spend/credit-cards/costco-american-...
I'm skeptical, since "cash advance" usually has its own stipulations regarding interest rates, and is capped below your credit limit. All for, what, $150 annually per $10,000 in arbitrage? That's an hour of work for many people here.