SOMEONE has to pick the blackberries/mushrooms/garlic leaves/nettle tops and package them for sale. They have to be paid a wage, and the premises have to be heated and lit and the rent/mortgage paid. In fact, the blackberries are probably cultivated ones (no thorns, easier to pick) The nettles ar garlic may be picked by someone who then sells them to the farm shop - charging for their time and effortt- and they have to choose patches where dogs haven't peed on them, and wash them before packing them.
As others have said, anyone can pick stuff in places open to the public, but if someone else picks it, they are entitled to recompense. That is the root of "the economy", at least until we achieve Utopia, when everyone does everything for no reward.