It may be the comparative costs (fees/discounts) in the AMEX transactions processing costs?
I just made a test buy on one of our websites with a prepaid VISA card I have -- not a problem. (USA issued Kroger kpf prepaid.) Shows what a crock of shit it is that a payment card proves any age verification -- I could have given that card to my kid ... Here's a card with $100.00 on it go play on the internet while your mommy and I are busy.
