[kragen]

This is a fully general criticism of competitive markets and is not limited to supermarkets. It applies with equal force to farmers, grain elevators, shippers, fertilizer makers, and (as you do note) the supermarkets' customers. You might argue that it applies with greater force to Monsanto (because they have government-granted monopolies on particular seeds) and to supermarket consumers (because they are not buying supplies for a product they are selling and can therefore choose to pay more).