You start to go in that direction you can easily say why would he let ANY injustice happen? Some things you have to let go of when dealing with superheroes.
Tony should have called the Avengers in IM3. Cap should have called the Avengers in Winter Soldier. Well akshully ad infinitum. And that’s not to say that you can’t or shouldn’t aim for a certain amount of “realism” with the genre, but some things can’t be due to the inherent nature of the genre.
You can’t compare Tony Stark to Superman.
Superman is the most powerful person on the planet in story. There is NOTHING that can stop him.
Superman is meant to be a non political character. That’s why the stories are kept to Superman fighting aliens, stopping disasters or taking down Supervillains who attack Metropolis.
The moment you have the character intervene in a “war” because he wanted to “save people” which is actually a valid/heroic reason then the character has to go down the rabbit hole of stopping ALL WARS to “save people” which logically leads to a Superman takeover storyline.
This is why the writers DON’T write Superman to do this in the first place.
Superman is a hero - he hears people are getting/going to be killed in war and intervenes because he has the power - Superman stops the war and saves people -
But now Superman decides he won’t intervene in any more wars despite his initial reasoning being that people are getting/going to be killed? Now Superman looks unheroic and not like a hero at all.
Good for you..... 
