How so? Isn't the tipping point of an antitrust suit a lack of competition and/or a monopolized position? Apple doesn't have that. They can do whatever they want in the confines of their own store.

They have close to 50% of the market in the US and I don't think a strict majority is required to be judged a monopoly.

You simply can't ignore iOS if you want to have a successful smartphone app business. That fact alone is enough in my opinion to consider Apple a monopoly and to take action to force them to play fair on their own platform.

I think an obvious comparison could be drawn with the movie industry of the 1940s, when the studios were forced to divest their ownership of the actual theaters their movies were shown in.

Abusing market dominance in one sector in order to shut out or outcompete other providers in another sector is the classic justification for antitrust action.