I think a good criterium for antitrust applicability (which the article implicitly mentions) is potential for damage: Even if a company doesn't hike prices, cut quality or blocks customers right now, how easy could they do this and how large would the damage to society be?

I think by that metric, Amazon (and others) should ring some alarm bells: If Jeff Bezos woke up tomorrow with an irrational hatred of redheads, he could immediately decide that his company should stop doing business with any redhead and denand that any partner company does the same - or they, too, will be kicked off Amazon. That would probably put redheads at a severe disadvantage in day-to-day activities pretty quickly.

Another thing I haven't seen discussed so far is creating sub-markets within your product. E.g., in the market for smartphone apps, Google and Apple are not large competitors or even monopolists, they are the government. They have full information about each market participant, can subject all participants to arbitrary regulations and instantly sanction players who don't confirm. Except, unlike an elected government, they are not required to justify the regulations, make them fair or even make them completely public.

Im kind of surprised such "sub-markets" seem to be completely unrelated so far.