The law defines practices that, while they happen in a marketplace, are anti-competitive and thus problematic. It is not anti-competitive to own the gas station next to the airport and charge a premium for gas. It is anti-competitive to buy out surrounding gas stations and close them to extract higher prices, or to conspire with competing stations to coordinate pricing.
Single-firm anticompetitive behavior, as defined by FTC: (https://www.ftc.gov/enforcement/anticompetitive-practices)
Single Firm Conduct:
"It is unlawful for a company to monopolize or attempt to monopolize trade, meaning a firm with market power cannot act to maintain or acquire a dominant position by excluding competitors or preventing new entry. It is important to note that it is not illegal for a company to have a monopoly, to charge “high prices,” or to try to achieve a monopoly position by aggressive methods. A company violates the law only if it tries to maintain or acquire a monopoly through unreasonable methods."
Horizontal Conduct:
"It is illegal for businesses to act together in ways that can limit competition, lead to higher prices, or hinder other businesses from entering the market. The FTC challenges unreasonable horizontal restraints of trade. Such agreements may be considered unreasonable when competitors interact to such a degree that they are no longer acting independently, or when collaborating gives competitors the ability to wield market power together. Certain acts are considered so harmful to competition that they are almost always illegal. These include arrangements to fix prices, divide markets, or rig bids."