A point that often gets neglected in discussions about tone policing is that it can be okay for someone to express something angrily and also okay for others to choose not to listen to it for that reason. These aren’t contradictory, and there are many situations where they’re both true.
There’s an inherent intimacy in listening to someone express strong emotion and it’s always permissible to opt out of intimacy.
This is definitely the correct personal attitude to take: to neither shame the angry/acting-out person, but not to give them undue sway either.
However this becomes a real problem in a larger social situation. Anger (like a smaller form of violence) is an extremely effective tactic for getting people to agree with you. It is so effective that we have basically banned it as a typical rhetorical tool, because both emotional and physical violence overwhelm the virtues we normally prize in conversation: accuracy, fairness, probity, etc.
When you give people “permission to be angry”, you are often handing them the keys to social victory. You may personally resist their anger, but most other people will say “this victim who is screaming about their pain needs to be placated, and your dispassionate defenses of yourself can go hang.” When one side of a dispute is allowed to be angry, and the other isn’t, we are choosing a likely victor. This is why we deter that with “overly angry people automatically lose” heuristics.
(This does not just apply to the obvious social justice circumstances, but also in the reactionary racist environment that preceded them. White people could be angry, black people couldn’t be without looking threatening, and so white people would win arguments like “how can you note allow the death penalty for this type of monster?”)
Ideally, you should have omni-compassionate environments that listens to the emotional rawness in everyone’s pain, but then makes its judgments coolly. But we can’t always (or ever) get that. In which case “no one can be publicly angry” is a rule that leads to better results than “some specific people can be publicly angry.”
This is very true.
It is also worth noting that “[only] some specific people can be publicly angry” isn’t the only failure mode here. Even if your community isn’t doing that, even if you genuinely allow everyone to be emotionally expressive, you’ve effectively turned almost every dispute into a contest of “who can most convincingly perform anger and pain?” Down that road lies a lot of terribleness.