Same reason any company can't set whatever they want. Vast majority of companies aren't deemed monopolies so this doesn't apply to them but this restriction holds over them once they grow to a certain size nonetheless. Even the most ardent capitalists/free market advocates agree that monopolies have to be regulated by the government.
The real questions are what makes company a monopoly. You can always argue you aren't a monopoly but it's what convinces people that makes most sense and many people are beginning to be convinced that Apple/Google etc are monopolies in certain markets.
