To understand why, consider how little investigative journalism the BBC produces. Certainly, when the powerful have a story to tell, they turn to its specialist correspondents, not least because they can be assured of intelligent and honest coverage. But the BBC leaves the breaking of politically sensitive scandals to others. Even Panorama, the one programme on BBC TV that has the freedom to investigate, is a feeble affair — a thing of shreds and patches. Typically, it does not reveal, but adds details to stories already in the public domain — the death of Baby P, the collapse of RBS, how Sir Allen Stanford fooled the English cricket authorities. Its one undoubted scoop in recent years was an undercover investigation into the mistreatment of the elderly in care homes. The documentary was good journalism, but not contentious journalism because no one apart from sadists wants the elderly abused. And in that distinction between the good and the contentious lies the danger.

Broadcasters must struggle to be impartial and to appeal to everyone. Investigative journalists take it as an insult if you accuse them of fair-mindedness, and have no desire to be everyone's friend. Their partiality is a professional blessing because it leads them to follow leads neutrals would ignore. A conservative is more likely to uncover the misuse of public money because she is already convinced that the civil service is a parasitical imposition on the taxpayer. A leftist is more likely to bang away at multinationals because he already believes that they are sinister conspiracies against the public interest.

Neither will always find what they are looking for, often because there is nothing to find, but their convictions drive them on until they get lucky. When they expose a genuine scandal, BBC reporters check facts and canvass opinion. In other words, they follow up, but they do not initiate.

I am not criticising. BBC correspondents cannot be partisans. BBC bias is shocking when it occurs because it affronts the core values of the corporation and undermines its straightforward journalists. In any case, the present division of labour between the politicised private press and non-combatant public broadcasters works well enough. Unfortunately, it cannot last because of the collapse of private profits. Investigative journalism is the first to suffer because it is expensive, particularly in Britain where newspapers face a rapacious legal profession and a judiciary that makes no effort to disguise its hostility to freedom of speech. As the internet wrecks newspapers' business models, editors will back away from risky stories, leaving the BBC exposed.

Think of the consequences of a future when the press is out of money. BBC correspondents might propose that they should initiate politically controversial investigations because they at least have the resources to fund them. You have only to envisage the scenario to guess the answer they would receive from their superiors. The Telegraph could publish the details of MPs' expenses because it is not beholden to the state. The BBC would never have broken the story. If it had, it would have learned about the relationship between paying the piper and calling the tune in double-quick time. The politicians would have blamed the corporation for their discomfort. Their disgrace would have been its "fault". 

At present when politicians complain, the BBC can tell them that its journalists are merely covering what all other media organisations are covering. It retains its independence to report accurately by finding safety from its paymasters in numbers, like a potentially vulnerable animal hiding in the herd.

As numbers dwindle and herd immunity declines, we look as if we will have a weak private press and politically vulnerable public-sector broadcasters. Those with expenses and other secrets to hide may have a far easier ride from the media in the early 21st century than they experienced in the late 20th.