In the market for personal computer (PC) software, Microsoft is the dominant firm with a 90 percent market share in the market for operating software. Microsoft’s market share in the office productivity software market is just over 95 percent. By the standard rule of thumb for gauging market power (a market share of 70 percent or more), Microsoft possesses this power in two separate markets.

Why might it be argued that Microsoft would not necessarily be prosecuted under the Sherman Act?