It's only allowed in one direction, but it's effective in many.

Violence is the reason slavery ended in the US. Violence brought us civil rights laws. Gay rights. Women's rights. Labor laws. Environmental protection laws.

Every right granted by default to white Christian gentlemen at the founding of this great nation had to be taken in blood by everyone else. That's just how America is. It cannot be trusted to live up to its own standards except at gunpoint.

When, where and how violence is justifiable is a different question, of course. But the premise that "Naturally, violence is never an answer, nor is it a politically effective tactic" is simply false. If violence were politically ineffective, authoritarian states wouldn't use so much of it.