Anti-trust law requires having a monopoly(which Apple doesn't have) or acting together with other companies. So in this case it's probably legal.
EDIT: that's basically the opinion of Keith Rabois, who litigated several anti-trust laws.
https://www.quora.com/Why-was-Microsoft-punished-for-violati...
If we use cars as an analogy, we require car manufacturers to allow service by other mechanics, but we don't require the same for phones (yet). Car manufacturers do not have monopolies.
At some point, asking, "Does Apple have a monopoly?" becomes the wrong question to ask.
True, but that required a new law, and sure a new law could also be deployed with regards to Apple.
Lots of android in the market but sold to people who couldn't tell the difference with iOS and wouldn't even know how to buy an app.
I don't think the government could force them to keep an app on their server and continue to serve it to consumers running their software if they didn't want to.