One of the most frequent causes of anger is the feeling of injustice. When one perceives that injustice has been done (especially against oneself), it fuels anger. This is an atavic reaction, baked into many species as can be seen from experiments on monkeys.
Now you might say that being able to react in a more cerebral manner than a monkey is sign of humanity ...
But one can also think that overriding natural impulses is also disturbing the great balance of nature. Also, from an evolutionary point of view, one has to assume that that chain of causality (injustice => anger) has been selected for a good reason ...
I think the original authors were trying to get at the idea of mastering the self before trying to master society.
Injustice certainly brings about anger. The question is, whether anger itself is enough to justify retribution. Anger is an emotional response and when acting only out of emotion, many mistakes might be made.
After removing oneself from a knee-jerk emotional response we can evaluate the situation better. If, after the anger has passed, we still feel it's appropriate to fix a perceived injustice(I prefer the term imbalance) then we should act, but anger in itself is not a legitimate justification.