> I’m not sure why people have come to the conclusion that certain emotions must be entirely avoided
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.
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.