Let's suppose that a reader, miffed by something I've written, spots me walking down the road and gives me an admonitory shove in the back sufficient to knock me over. He or she might consider us both unfortunate if, in the next few minutes, I died of a heart attack, but could surely not complain if arrested, charged and convicted for common assault.

The shove on the newspaper-seller Ian Tomlinson was almost inevitably captured on camera, and even more inevitably compared by a member of the Home Affairs Select Committee with the shooting of Jean Charles de Menezes in 2005.

This comparison is wrong. The officers who killed Mr de Menezes believed, however wrongly, that he was an immediate danger to all those around him. But what was it about Mr Tomlinson's behaviour last week that could possibly justify him being pushed to the floor? Was the officer afraid that this very ordinary and - at that moment - isolated man was suddenly going to metamorphose into Super-rioter, rallying the forces of international anarchism (all 25 of them) and bringing down the towers of Mammon?

Or was the policeman fed up because, hands in pockets, Mr Tomlinson was not clearing the area with quite enough dispatch? It really doesn't matter. The public will cut the police some slack for making mistakes, even terrible ones, in desperate circumstances. It will not agree to being assaulted for the crime of being mildly annoying.

Unfortunately, the mentality is a familar one. It was encapsulated by the chairman of the Metropolitan Police Federation, Peter Smyth, when he said that “sometimes it isn't clear, as a police officer, who is a protester and who is not... I know it's a generalisation, but anybody in that part of the town at that time, the assumption would be that they are part of the protest. I accept that's perhaps not a clever assumption but it's a natural one.”

Let's decode this. If you go to a demonstration at which, according to advance hype, there may be violence, you can expect to be treated as though you were threatening violence yourself. Or, as a senior police officer said to me eight years ago, when I was trying to escape a police “kettles” (penning demonstrators into a contained area): “You are a volunteer, mate.” In other words, all of us present were the enemy and we shared a collective guilt.

Over the years I've seen this mentality many times at demos. And it is always counter-productive, often dangerous and, on very rare occasions, lethal. The officer and his colleagues should realise that it isn't us and them, just us and us, and then fess up. Coming forward, as he now has, is an important first step.

Times Recommends

• A dose of defeat is just the medicine
• Protecting ourselves from Eurocreep
• After the summer of LUV, it'll still hurt