Some small shops do and they can do this, most will want you to spend £5 or so to make it worthwhile. No card transactions are technically free you know
Exactly this.
Debit or Credit Card transactions typically incur a charge for the store owner.
Credit Card charges are typically higher (due to "better server" they add i.e. the fraud checking and higher amounts of money they'll quickly refund etc)
It's more common to see Credit Card charges passed on to the buyer because of this.
Debit Card transactions still have a fee and for smaller businesses this is typically a fixed amount, up to a certain amount (50p for all transactions up to £100 say)
So yeah, a 50p charge if you're buying a 50p can of coke means they loose money if you pay by card.
As such, all retailers are allowed to pass this charge on to customers and most small shops typically say that they'll pass that charge on to you, for the privilege of using your card, unless you spend more than £5 (but sometimes up to £10)
It seems fair enough to me and I'll admit to typically either:
1) Using the cash machine down the road before going to the shop
or 2) Spending up to £5 to top up on biscuits or some coffee
Don't really have a problem with it since I know the ins and outs of it.