This is interesting because it seems like the DOJ's definition of monopoly is changing, which I think is a good thing. Since the late 80s (roughly), antitrust litigation has only been brought on the basis of a monopoly's negative impact on consumer prices (i.e. does the existence of a monopoly cause the consumer to have to pay higher prices?). That ignores monopolies that impair the ability of smaller companies to en…
In the European space, protection of smaller business' ability to compete is factored in. But the US space, in general, has not cared if smaller businesses can compete and tends to take a laissez-faire attitude towards business-on-business market pressures. It's the effect of the market on consumers that's paramount.
To be honest, I'm not sold on the European-style philosophy. Guilds traditionally empowered tradespeople, but they also had a reputation for holding back innovation and punishing new ideas that didn't fit the existing paradigms. Amazon, for example, has made book-buying so cheap nationally that individual bookstores have trouble competing. Amazon's other failings aside (which should be addressed by sweeping employee / labor protections, not by breaking up Amazon): should Grandma's books be more expensive in general so the local bookstore owner gets to enjoy the lifestyle of self-business-ownership to which he is accustom? Why / why not?