Imagine Paul Chambers sitting in a pub, boasting about a wonderful woman he was hoping to make his own. As he describes how he will fly from the East Midlands to her home in Northern Ireland for their first date, he catches a glimpse of the television news. Bad weather has closed the airport.

"Crap!" he bellows at the screen in mock fury. "Robin Hood airport is closed. You've got a week and a bit to get your shit together, otherwise I'm blowing the airport sky high!"

By definition, a pub is a public place but no one takes any notice. His friends laugh at the lame gag, then forget it. A busybody by the bar overhears him, but the smile on Chambers's face tells him that he is joking. In any case, the busybody reflects, terrorists have attacked airports for many reasons and delayed passengers have found all kinds of ways to express their frustration. But no one has ever blown an airport sky high because snow has closed its runways.

Instead of joking in a pub, however, Chambers joked on Twitter. His case is a cautionary tale to all who do not realise that the new technologies are Janus-faced. As their boosters say, they can indeed allow information sharing and new opportunities for collective action. In a small way, Paul Chambers knows it too. He would not have met his girlfriend if they had not both been on Twitter.

But Chambers has learned the hard way that the web also allow spies, dictatorial and democratic governments, cyber-bullies, blackmailers and every variety of prod-nosed, pinched-cheeked bureaucrat to practise surveillance on a scale that would have made their predecessors gasp. An official at Robin Hood airport searched Twitter to read what others were saying about his employers. He saw Chambers's tweet and passed it to the security manager. He decided it was a "non-credible threat".

But the regulations said that he had to pass all threats – credible or incredible – to the police. Orders are orders and he followed them. Detectives arrested Chambers at his workplace in front of his employers, who promptly fired him. The police also decided that the "threat" was nothing of the sort. The knowledge that he was just a guy who wanted to see his girl did not stop them from calling in the lawyers, however. The Crown Prosecution Service duly went for Chambers. It persuaded Doncaster magistrates to find him guilty of sending "a message of a menacing character".

Chambers found some relief from the absurdist drama when, finally, he flew to Belfast to see his new friend. They got on so well that they settled down as a couple and Chambers took a job in Ulster. The week before his appeal was due, he warned his employers that the media were still interested in the bomb threat that never was. Belfast is not the best place to use the word "bomb". They sacked him too. He has gained a criminal record, lost two jobs and is now on the dole. All for making a joke.

The "Twitter joke" affair is a cause celebre because it punctures the comforting illusion that Britain is a moderate country. As I have said before, no one in authority from Doncaster magistrates court to the offices of the director of public prosecutions has shown they possess a sense of proportion or a glimmer of an understanding of how social media work.

I suspect, however, that a deeper worry underlies the protests. The web is making what was local global. It makes that evidence of faults, which once would have been forgotten, permanently available to the malicious and small-minded. Before the web, if a student had celebrated to excess after finishing her exams, a friend might have taken an embarrassing picture of her. But it would have stayed in an album or attic.

If her celebrations had been so excessive and the police had charged her with being drunk and disorderly, a public account of her debauchery would exist in a criminal record or newspaper cutting. But they would have been buried in dusty filing cabinets and unvisited archives. Old technology ensured that public records were almost private.

Now, potentially anyone with access to the web can find embarrassing photos, evidence of minor crimes and foolish Facebook updates or tweets. The gawky girl is no longer bullied only by her vile classmates but vile strangers too. Iranian revolutionaries learn that the apparently liberating web allows the secret police to monitor them and discover their contacts. An employee such as Adrian Smith, a Christian working for the Trafford Housing Trust, finds that opposition to gay marriage on his Facebook page allows his employers to lop £14,000 from his salary. They do not try to argue with him or warn him that he must not let his prejudices affect his work. He has uttered "controversial" opinions his colleagues do not like and that is enough to damn him.

The utilitarian philosopher Peter Singer says we should welcome the exponential increase in the possibilities for surveillance. If others know more about us, we know more about them. We will move to a free and open society. We will be less ashamed of our secrets and less censorious of our neighbours. Disclosure will bring the greatest happiness to the greatest number.

Although many want to share details their ancestors would have regarded as private, Singer does not understand that western democracies remain hugely hierarchical. The managers of private and public bureaucracies justify their elevated status and salaries not only by attempting to run efficient organisations (a task that is often beyond the poor dears) but by monitoring and intimidating those beneath them. The web gives them unprecedented power to police information that once would have been beyond their reach.

Paul Chambers's appealed to the High Court last week. Robert Smith QC, for the CPS, kept saying that the tweet was a public statement. But it would have been just as much a public statement if Chambers had made it in a bar, or at a party. The small but devastating difference is that he wrote on the web that never forgets and can be accessed by anyone who wants to find incriminating evidence. It is a difference that changes everything.

If their naive questions were a guide, the High Court judges did not grasp the transformation either. Soon, we will need judges and lawmakers fit for the internet age, who know that just because the web makes people's information publicly available does not mean that public or private authority has a right to punish them. In fact, we need them now.