Visa and Mastercard as associations haven't banned the funding of gambling sites with a credit card. Discover, American Express and PayPal have.
Unlawful Internet Gambling Funding Prohibition Act (HR 556) was passed to keep gamblers from funding with credit card accounts. Online US gamblers have then sought to add funds from Canadian companies such as NetTeller.
Mastercard and Fleet along with Visa and Travelers were sued for illegal collection of an unlawful debt. John W. Kindt and John K. Palchak wrote a terrific book on this subject called, "Legalized Gambling 's Destabilization of U.S. Financial Institutions and the Banking Industry". One author is an Illinois law school graduate and the other is a credit union analyst.
The simple solution is to erect more barriers to fund internet gambling accounts. Visa / Mastercard would push this cause along nicely if they refused gambling merchant charges.