There's an easy way out - they simply refuse to take the credit card! The merchant agreement is between the merchant and Visa (or Mastercard, etc) - the consumer does not have standing to challenge or enforce that agreement. If Visa wants to allow a merchant to violate the agreement (and in my experience they have explicitly told me this), they are free to do so.

So, yes, merchant agreements generally allow NO minimum or maximum purchase limits to be required by the merchant (try enforcing that next time you're at a Mom & Pop liquor store with a "$5 minimum on Credit Card"!). But, NO, you can't make them stick to that.

That being said, I ordered a new Porsche last month and the dealer said, "I'll need 10% down - will that be cash, check, or credit card?" And I put $9K on my credit card without them batting an eye. So, it depends on the dealer.