Clearly businesses that accept AmEx are considered consumers for that purpose. This is not being debated here.

What is being debated is if there is a two sided market (two separate groups of consumers), should we add a NEW interpretation that requires harm shown for BOTH groups of consumers rather than just ONE group of consumers.

Yes, and this case was showing the price change for businesses that accept AmEx. T

Consumer protection isn't just about prices, and antitrust isn't just about consumer protection.

In United States v. Microsoft [0], Microsoft was found to have abused its dominance as the maker of Windows to compete unfairly against Netscape, which was free (as was IE), among others. The "browser wars" was Microsoft blocking entry into a market where the price was already zero.

[0] https://en.wikipedia.org/wiki/United_States_v._Microsoft_Cor...