A Bristol jury this week controversially acquitted the four accused of causing criminal damage to the statue of Edward Colston, the 17th-century slave trader. After the verdict, many people, from Jacob Rees-Mogg Leftwards, have reminded us that the jury system is a pillar of our liberties.

It certainly is. But instead of repeating this as a piety, one must state why it is so. It is not because citizens are well-versed in law: we are not, and cannot be expected to be. Nor is it because citizens should deal out punishments: judges, not juries, decide the sentence.

It is because our history teaches us that a semi-random selection of our fellows is capable of judging facts after hearing the arguments on both sides. Twelve such men and women are probably better at this than legal professionals, who are much remoter from ordinary people and likelier to become agents of state power.

That, at least, is what we in Britain think. In countries such as France, where the state is more revered and juries have only a marginal existence, they think our system is strange.

In a way, it is. I have never served on a jury, but I have talked to many who have. All of them tell me in general terms of imperfections – jurors too eager to reach a verdict in time for lunch, or who think they are engaged in detective work rather than deciding on the evidence presented, or who have such a horror of a particular crime (eg, paedophilia) that they too readily assume guilt.

But my friends usually report that the system corrects itself as it goes along, because the more conscientious jurors get the others to focus. None of them says we should get rid of juries.

Since juries matter so much, however, we need to ward off threats to them. The most obvious ones come from intimidation or corruption. In a very divided society, this is a big problem.

In Northern Ireland 50 years ago, terrorists threatened jurors who might convict them, and a jury dominated by one of the two communities would often be unfair to an accused from the other. So judge-only “Diplock courts” were introduced in serious criminal cases. This was undesirable, but necessary. Diplock courts were abolished, except in exceptional circumstances, in 2007, when the Troubles had clearly died down.

Such threats are not a constant problem in Britain today, thank goodness. But there is a growing tendency – the Colston statue case is a prime example – of court cases being used as political theatre. Little standing armies of protest, often loosely associated with Black Lives Matter or Extinction Rebellion, wander the country looking for trouble.

If they are arrested, they deliberately contradict themselves. On the one hand, they proclaim their innocence of any crime. On the other, they proclaim their courage in defying the forces of law.

If the matter comes to court, they turn up in considerable numbers with vigils, placards and (as happened this week) a good deal of noise in the public gallery. When interviewed by the media, they always claim they are enacting the will of “the people of Bristol” (or wherever).

Note how unfair this is to those who actually do represent the people. Marvin Rees, the elected (and re-elected) Labour mayor of Bristol, is the most senior black person holding directly elected office anywhere in the United Kingdom. After the acquittal of the “Colston Four” (all of them white, two of them originally from out of town), Mr Rees said that Bristol was not the place “to indulge your own fantasies of being a revolutionary”. Unfortunately, it looks as if it is.

The more the protesters spread the story, in court and in the street, that they speak for the community – with media and social media fanning such claims – the harder it becomes for a normal local citizen who finds him/herself on a jury to do the job properly.

It is alarming to have that level of attention and potential anger directed at the decision of which you make up a twelfth. You would be brave not to worry about being called a racist if you arrived at the “wrong” verdict.

Probably for that reason the judge, Peter Blair, directed the jury that the public excitement about the case was not relevant to their task. Their sole duty was to decide what were the facts.

One has to go carefully here. No one but the jurors themselves knows what was said in their room, and it is right that we should not.

The accused were undoubtedly present at the statue’s fall and enthusiastically participatory. One, Milo Ponsford, admitted turning up with 15-20 metres of rope, tying it round Colston, giving a signal that it was time to tug, joining in the pulling, jumping for joy on the fallen statue and messaging immediately afterwards: “Yeah, we pulled down the statue, ha, ha, ha.” But perhaps the jury felt the case that they had caused criminal damage had not been properly made out.

What one can say, however, is that counsel for the defence and the accused themselves used the court not to persuade the jury about the facts, but for another purpose.

True, they made half-hearted attempts at distinctions between being happy to pull down the statue and trying to cause it actual, positive harm, but their real message was that they had been “preventing further harm to the people of Bristol” (Ponsford), that the statue had been like “having a Hitler statue in front of a Holocaust survivor … if not worse” and constituted a “hate crime” (Sage Willoughby, another of the accused). Therefore, it was asserted, the jury needed to get on “the right side of history” (defence counsel).

The verdict of the jury suggests that it took such words seriously. The trouble is that such words do not express facts. They are words of polemic and politics.

Rhian Graham, another of the accused, told the court that it had been “poetic justice” when a black man had stood on the plinth from which Colston had been deposed. Perhaps, but it was not justice in the meaning of the word in a court of law.

If juries are to mete out “poetic” justice, then they will no longer be pillars of our liberties. They will have become auxiliaries of the sort of extremist politics that loves making trouble and cannot win elections.

I do have one smidgeon of sympathy with the accused, however. Jake Skuse, the fourth of them, noted in court that the police stood by during Colston’s deposition and did nothing. “How can I think it’s a crime?” he asked. He appeared to mean not only that he thought it the right thing to do, but also that the authorities were letting him do it.

I fear Mr Skuse is right that there is now an element of official permission in attitudes to such protests, which is why the Government is tightening the legislation in reaction.

This is permission not only in terms of policing, but in the attitude of the law itself. More than 20 years ago, the Macpherson report on the killing of Stephen Lawrence defined a racist incident as “any incident which is perceived to be racist by the victim or any other person”. Such a definition, and the related concept of “hate crime”, elevated an individual’s feelings over evidential fact.

This created a problem of which the Colston case is only the latest example – the criminal law turned into a political battlefield, with justice the loser.