All of the aquatic centres in the area I live keep their tanks in tip top condition. It only makes sense to make sure that what your customers are looking looks clean and healthy. I certainly wouldn't purchase anything from a tank that wasn't looked after properly.
Regarding the Oscars, unfortunately, many people buy Oscars and don't look after them properly which is why they get hole in the head disease. I don't think it's the fault of the shops, they probably already have the disease when people bring bring them in. I expect that many people buy Oscars when they are very young and then realise they can't cope with them when they grow very big, that's why you see so many of them in the shops.