The banks that issue commercial credit are doing so because of the Federal Reserve Act and legal tender laws (i.e. gubmint force)....They need to be held to the laws from which they derive their profits.
The real issue is not this symptom. The real issue is the incredibly fraudulent, unconstitutional Federal Reserve Act. Get rid of the Federal Reserve and I would be very happy.
But, that is another issue all together.
This comes down to contract law, not constitutional law or federal law.
Citigroup can include a term in their credit card agreement that they will not extend credit to a card holder for the purchase of a firearm. The credit card purchaser can then choose to tell Citigroup to go fuck itself in the ass with a 20-foot concrete dildo and move on.
Some lender out there will cease on this opportunity and make a great profit, at Citigroup's asshurt expense.
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