Actually they do because tender is required to complete a transaction if you don't use barter. It only gets dicey when a debt is involved, but normal store transactions don't normally constitute debts. Anyway, there are no laws in America that require someone to accept a bill unless a debt is involved (thus the words "Legal Tender for all debts, public and private").
In the UK, legal tender laws specifically list exceptions such that stores, city councils, and the like get relief from pester payments. You're expected to pay your dues with a reasonable spread of coinage and/or notes. In general, notes are always welcome unless they're too ungainly (say a £100 to buy £2 worth). Pound coins, again, are good in general. Pence coins, OTOH, tend to limit you to no more than certain amounts at a time for various coinage denominations.
Biting the hand that feeds IT
