This is a "can" vs. "may" question.
The merchant agreements do not allow minimums, but it is a contract issue only. Even if a consumer could convince the merchant bank to "slap the wrist" of the merchant, I doubt that the merchant bank, MasterCard or Visa would actually sanction a merchant.
The consumer should complain to the merchant and take their business elsewhere.
Al Miller, CRCM
Opinions expressed are my own and not necessarily shared by my employer.