Childish anger has an untamed destructive element. But the answer is to let anger grow and mature with the rest of your psyche. Not banish it and pretend it doesn’t exist. If you banish your anger, how could it possibly mature?
I don't think "avoided" is the right term for these teachings, but the emotion itself is generally not fruitful, particularly since it urges you to dwell on it. Injustice should be addressed because it's unjust, not because you're angry at the injustice. What does the anger add to the equation? Maybe motivation? Maybe it's important to explore why the injustice by itself is not sufficiently motivating.
Anger draws attention toward what’s important - for both ourselves and others. It says “hey! Something isn’t right here, and letting it slide wouldn’t feel right!”. It is a constant voice reminding you what you love and what you would sacrifice for that love.
There’s a lot of talk in certain communities about boundary setting. But I see boundaries as a natural part of the domain of anger. Boundaries are only hard to set if you either don’t hear your own anger internally, or don’t let that anger be expressed into the world.
Even if you know what matters to you, anger can still be a wonderful tool to communicate your desires and intentions to others. If a homeless person starts creeping on to my partner in the street, I’d almost certainly reach for anger when I tell them that they need to cut that out. It’s a powerful, direct way to speak straight from my lizard brain to theirs. Seems ridiculous to take that tool out of your toolbox.