I don't think anyone inferred that supermarkets are out to cheat people. However supermarkets are working with huge quantities. They specify the varieties of fruit and veg that are grown for them under contract, and frequently supply the seed. They choose varieties that do not respond badly to being transported over large distances and stored for what would otherwise be considered long periods of time. That is why they also sell fruit which is under-ripe and which they say can be ripened at home. Because it is firmer than ripe fruit it doesn't show damage so easily when being packed and transported, and can be held in huge storage facilities in a sort of 'suspended animation' . This is also true of potatoes and green veg - you have only to travel through the Fens and see the vast storage facilities that have been erected. This enables the supermarkets to maintain an almost constant supply of fruit and veg, regardless of season, and that is what some customers want. Some people want to buy broccoli, green beans and strawberries throughout the year, rather than when they are in season.
Smaller shops are supplied by growers who have different priorities - specialist varieties grown for flavour or other attributes. Because these are frequently grown locally to the customer they don't have to withstand the amount of handling and damage caused by transport, and because the customer who buys fruit and veg from smaller independent shops usually understands about seasonality, they don't have to provide broccoli or green beans 52 weeks of the year, but only when they can be grown in season in the UK.
And I definitely know what I'm talking about - a close member of my family is one of the UK's largest supermarket suppliers of potatoes and vegetables - and another close member of my family is involved with running a Farm Shop specialising in locally produced and specialist products.