[kstenerud]

"Section 1 of the Sherman Act is violated when an unreasonable restraint of trade within an oligpoly results from a firm's course of action which serves to exclude competition from that market."

- United States v. Griffith, 334 U.S. 100 (1948)

There have been a number of bench decisions based on barriers to entry and market entrenchment to prohibit this sort of behavior by oligopolies, even if the Sherman act didn't specifically mention them.