When a manufacturer makes a retailer sell the product at the price designated by the manufacturer, by increaseing the wholesale price of a product or stops supplying the product to a retailer that does not sell the product at the designated price, this is called "resale price restriction."

As you can see in the above illustration, if the manufacturer stopped supplying the product to Retailer A, the product would be sold for 3,000 yen, as designated by the manufacturer, at all other retailers.
Consumers would then not be able to choose a sales outlet based on price, and would be forced to buy the product at a higher price than the low price at which it should have otherwise been sold. This would decrease consumers' benefits. Such resale price restriction is prohibited as an unfair trade practice.
However, such copyrighted products as books, magazines, newspapers and music CDs are exceptions, so they are not subject to the prohibition of resale price restriction.