Microsoft does the same as Apple, it makes money off micro-transactions and the ability for its competitors to sell on Xbox/Microsoft store, use trade dress, and be included in those hideous green media cases.
Last I checked Epic, Steam, Origin, uPlay, Google, Amazon, SetApp, most retailers, etc... do the same practice to varying degrees. Shrewd business practice != injustice/anticompetitive practices, just because a company has a large market share or is personally abhorrent to you.
Civil suits are also one of the mechanisms by which government regulation is enforced. The alternative to that would be the government inserting itself into private relations even when there is no dispute between the parties; which is even more heavy handed (albeit somewhat justified).
It is a weird quark that you often have to violate the terms of a contract in order to get standing to challenge it; but that is the procedural rules we have.
They had standing the whole time, albeit with a limitation to arbitration at first instance for certain claims, which may have been bypassed on certain grounds anyway.
The temporary restraining order (injunction) application claims they want to be able to post Fortnite on the App Store without scrutiny from Apple and lastly that they regain access to a working developer account for unreal engine.
I recommend watching the virtual legality series on this topic which might interest you on the ins and outs of the legal dispute. The latest episode covering Microsoft is here: https://youtu.be/-jXJjllz00I
Edit: spelling.
I think Epic has been smart to do a lot of PR on this. They need public support since this will become political. I'm surprised that Epic hasn't filed in the EU too.
However, Apple is also one of the largest companies of USA and a quite successful one. At that size they must have considerably good lobbying efforts, political class might also have connections to Apple through shares etc. They might go and say that enabling third party vendors will cause thousands to lose their jobs at Apple. They basically got a huge amount of political influence and I’m sure they will use it for this case.
How much political influence Epic games have? Zero? Perhaps not zero but definitely not at the level of Apple. Hence, this case might not be a straightforward win for Epic.
It is good though, perhaps some of the fortnite players will understand why antitrust laws are needed and perhaps influence the justice system once they grow up.