A better one that I can think of: Imagine a violent clash between protesters and counter-protesters. To me, that is potentially an appropriate use of tear gas, because things have escalated to the point where people are being harmed.
I think, though, that, what's interesting with both my and your hypothetical, and markedly distinct with what's been happening in the news lately, is that we are not talking about a simple face-off between protestors and police. Perhaps that's cultural DNA? I would guess that virtually every natural born citizen of the USA studied the Boston Massacre in history class, and is consequently at least somewhat aware that violent retaliation against civilians - even an angry mob - doesn't have a great track record of actually making things better.
> Let's look at an unstated major premise here: That it's imperative to achieve the result in question.
I tried really hard to create the perfect hypothetical situation to discuss the correct use of force instead of discussing if the use of force is correct.
I failed pretty badly it seems and this time jnlike a number of other times I can't see why.
At least you were polite, have my upvote :-)