Oh god, only one thing?!?
How about:
- they are preventing other, smaller stores from succeeding in business
- they waste loads (as an above post said)
- they are swamping the market in as many areas as they possibly can
- they encourage spending by having BOGOF offers (as do many other supermarkets, I know)
- they encourage out of town spending, thus preventing shops in smaller areas succeeding
- they mess around small suppliers as their produce isn't always to the 'perfection' they require (eg not perfectly round oranges, apples with small marks, etc) and can often change their mind at very short notice, so leave producers stuck with produce
Sorry, couldn't choose just one