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USA, where people are afraid to heckle.

I would rather put it like this:

USA, where people already are afraid to heckle.

It is probably the alarming symptom of the advanced level of fear instilled in the people.

Please explain how it's at all useful to heckle?

Heckling is probably the lowest form of displaying your disagreements with the person doing the talk.

Heckling is one of the oldest forms of political dissent. If one can't handle heckling, one shouldn't be a politician or any kind of public figure.

That doesn't make it acceptable / useful as per my question.

It's very useful. We're still talking about it.