It works like a normal bicycle. If the tires are rolling forward, there is a freewheel so that the pedals don't need to be moving (or can be spinning backwards). However, the freewheel only works going forwards. If the freewheel worked both ways, then there would be no way to transfer power to the wheel.
Therefore, any time he his coasting backwards, he needs to be backpedaling to prevent the back wheel from locking up. This actually makes it significantly harder. When I used to own a bmx bike as a kid, I could do a 180 and coast backwards by standing on the pegs. However, with my feet on the pedals, I could only coast backwards a half foot or so, and either the back tire would lock up, or I would lose my balance from trying to pedal backwards.
It is easier to keep your balance pedaling forwards, because there is resistance from the wheel. Pedaling backwards there is no resistance, so if you are standing on the pedals (as you always do in trials, notice the bike has no seatpost) you have to use your muscles to pedal smoothly.
It is odd that the pedestrians don't seem to care. Maybe they see him as some young punk hoodlum, like how many see skateboarders.