https://www.ftc.gov/tips-advice/competition-guidance/guide-a...

The TLDR is that if the refusal to deal with a competitor helps maintain the monopoly, then it might violated anti-trust law.

Here are the main points:

"In general, any business — even a monopolist — may choose its business partners. However, under certain circumstances, there may be limits on this freedom for a firm with market power. As courts attempt to define those limited situations when a firm with market power may violate antitrust law by refusing to do business with other firms, the focus is on how the refusal to deal helps the monopolist maintain its monopoly, or allows the monopolist to use its monopoly in one market to attempt to monopolize another market."

"For instance, if the monopolist refuses to sell a product or service to a competitor that it makes available to others, or if the monopolist has done business with the competitor and then stops, the monopolist needs a legitimate business reason for its policies."

There are absolutely cases where refusing to do business with competitors, can violate anti-trust laws. It says so right there, on an official government page related to this.

This, and similar things were happening, related to railroad services and prices, related to standard oil, which is why I brought up that example.

But that government page is a more general explanation of the problem.