It's essentially the same as putting a price floor below which a reseller is unable to sell. Because they cooperate together to charge a high price, this eliminates competition and creates a monopoly of the reselling corporations. Dealer competition is limited by a minimum price contract.
Antitrust laws in the United States restrict a variety of anti-competitive behaviors, including RPM, in addition to forbidding mergers. Antitrust regulators scrutinize any tactics where a single corporation controls a significant portion of the market. In such a situation, RPM might be useful.