Natural monopolies are not illegal. It is illegal, though, for a business to conspire with others to drive its competitors out of business in order to create a monopoly. Unlawful Relationships With Competitors Arrangements between competitors are called horizontal arrangements under antitrust law. Some of these arrangements run afoul of antitrust laws. Generally, the agreements businesses enter into are unlawful only if they are for the intentional purpose of restraining trade or gaining a monopoly in the market.
Some agreements are per se illegal, however.
A per se illegal relationship with a competitor is one in which no anti-competitive intention must be proved. The arrangement itself, no matter what the purpose, is against the law. Collective Refusal to Deal A collective refusal to deal, also called a group boycott , is a concerted decision by competitors not to do business with another business. Unlike a single company's boycott, or a boycott by consumers of a particular business, a group boycott is unlawful because it has the effect of restraining freedom of trade. Joint Ventures Obviously, a joint venture is not in and of itself an unlawful business action.