As a fish keeper I don't see it as cruel UNLESS the shop isn't filtering the water to the correct level or even beyond. The fish are doing what comes naturally and despite popular opinion do not need to be starved to do the 'job'. I have kept various fish over the years that despite regular feeding still rasp relentlessly at the wood in the tank, the glass and some of them would do it to other fish if they didn't have the correct things in the tank to do what they would do in the wild.
In all the years I have kept fish I've yet (touch wood) to get any infection from water changes or having to handle the occasional sick fish. The tetras and danios love to come to your hands and nibble around your nail folds if you keep still for a moment no matter how clean my hands are.
Interestingly I did observe once when watching through the fish spa window, one tank where the fish weren't so keen and it was a lady with obviously well maintained feet. I wondered if she might have had scented cream on her feet. In my experience there are fish that will seem to eat anything but watch them more closely, flaked foods are a good example, certain flakes attract some fish, some attract others. Fish won't generally eat something that isn't agreeable.
All that being said, the place I went to in Derby did check the clients feet for open sores and signs of infections. The fish themselves aren't capable of actually breaking through the skin, they can only rasp at the dead cells and I wouldnt be expecting much change after just one visit.
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