Earlier quoted context omitted.
Market share is the differentiator in both scenarios.
So anti-competitive actions/feature/function is only a problem once you reach a certain market share?
There's nothing wrong with refusing to interoperate with competitors. The market often works to take care of that to some extent.
However, when you have one player which is manifestly more powerful than the competition, you don't have a fair market. The whole reason we have anti-trust legislation is the recognition that this isn't desirable, and that businesses with too much pressure can exert influence which is harmful to the market in the long term.