If a company, either individually or with others, tries to force its competitor out of the market or deter the entry of the competitor in order to dominate the market, this act is called "private monopolization".

As you can see in the above illustration, when a company that monopolizes the market offers rebate to its counterparties to meet their trade volume, aimed at eliminating rival companies that have recently entered the market, the counterparties will deal only the products of the monopolizing company. If this happens, new entrants that have superior products in terms of price and quality are forced out of the market.
Since the monopolizing company has ousted its competitors, the company stops making corporate efforts to sell better, quality products at lower prices to consumers. This behavior eliminates consumers' benefits. That is why such private monopolization is prohibited.
Of course, if a company monopolizes the market as a result of fair competition, it does not appear any violation.