A while ago I posted to a thread about dining programs (AAs to be specific), and mentioned that several restaurants have a minimum amount to pay by credit card (and thus get miles). My response was deluged with posts saying this clearly is a violation of the merchant agreement, as the merchant must accept the credit card transaction in any amount.
Nevertheless, I still see many merchants (particularly smaller restaurants and other businesses) posting a $10 or $15 minimum credit card charge, and apparently getting away with it.
Regarding this particular thread, I know some people who don't wish to carry any cash on them whatsoever, and they pay for the smallest purchases with their plastic. I've made a few charges on items less than $5, and cannot really recall any rude remarks about it.
Once in the 15 items or less line at the grocery store, I remember someone in front of me putting a 38 cent charge on their Platinum charge card for an onion . I personally would be embarrassed to charge something for such a tiny amount, but the cashier professionally handled the transaction and said nothing out of line to the onion's buyer.