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.