When it's endorsed by the government, of course.
I couldn't help but laugh at the irony of this.
First of all, we have an instance where riot police used LRAD, tear gas, and mace on college students celebrating at an annual party:
And then, we have an undoubtedly much larger and, presumably, more wild (as evidenced by people hanging off of light poles) party outside of washington DC, celebrating the death of Osama.
So where was the riot police there? Call me crazy, but that crowd looked far more wild (and FAR stupider) than the college crowd they attacked.
Oh right... Government endorsed idiocy prevails.