It's simply part of being a company that happens to have a dominant position in a market or markets. It's ultimately up to regulators and courts to decide if a "dominant" company is engaging in anticompetitive behavior and what actions, if any, need to be taken to try to bring more balance, fairness, etc. to the market(s) in question. These are reasons why countries/regions have antitrust laws and regulations.
Well, in this instance, there has been no decisions by any court that determined iOS or Android are guilty of anything. This commission just went ahead and came up with a set of rules to destroy iOS.