Why, when it's committed by a police officer. Here, take a look at this video of a Denver police officer grabbing, punching and body-dropping a man talking on his cell phone: Video Link
Now, stuff happens, and every profession has those that fall short. And among real professionals, there are times even the best fail to live up to the standards set for them. So seeing a police officer lose his temper isn't really earth-shaking.
What you might find just a little bit....well, terrifying, is that this officer was given three days off without pay for 'submitting an inaccurate report.' No, he's not in jail for his unlawful assault. He's not even on indefinite leave, while the department waits for the indignation to die down. He's back out on the streets of Denver, presumably ready to do that to someone else's kid at this very minute.
And despite the outrage of Denver's citizenry, despite the publicity that's been given to this, despite the actual police camera video that shows the officer committing an assault on a citizen, the decision, as of this date, stands.
The failing of an individual trusted to police us is sad. But the failing of the system designed to oversee and check that individual is another symptom of having crossed the line between government and oppression.
No logical person would argue that this is right. The citizenry's being unable to change it is proof that we're no longer a government 'of the people', as the people don't approve.