You're right on that one. The merchant agreements all forbid adding on fees for use of credit and debit cards. They also forbid minimum charge but I see minimum $5 or $10 in mom and pop stores all the time.
That's probably because the fees for each transaction they're paying is probably around 20 cents per transaction plus 1 or 2 percent of the total.
So a charge for $1.00 would mean the fee would amount to 21 cents! Imagine!