AMEX doesn't care what I'm spending on so long as it's legal (obviously) and it's actually me doing the spending. Visa/Mastercard care far too much about how I spend money, which is frankly none of their business so long as I properly pay the bills (which I do).
AMEX is in the business of moving money between customer and merchant and performs their job exceptionally, so I'll always use them over Visa/Mastercard when possible.