One of the perks of wearing the robe of a judge is that you don't have to explain yourself. But when a judge sends to prison for three years-plus the man who pointed US tax collectors to billions of dollars of untaxed American wealth, who tore open the veil of secrecy surrounding Swiss banking, he ought to say why.

If Bradley Birkenfeld hadn't blown the whistle on his former employer, UBS, thousands of Americans would still be evading taxes instead of paying them on some $20bn (€13.8bn) they had stashed away overseas at the bank. Switzerland wouldn't have been forced to agree to make banking more transparent, and hundreds of suspected tax evaders wouldn't now be under investigation, spawning guilty pleas and multimillion-dollar recoveries for the treasury.

"Without Birkenfeld walking into the door of the justice department in the summer of 2007, I doubt as of today that this massive fraud scheme would have been discovered by the US government," Birkenfeld's chief prosecutor, Kevin Downing, told the sentencing judge in August. So why was Birkenfeld headed to a federal prison in Pennsylvania last week?

US district judge William Zloch gave no explanation in August when he sentenced him to 40 months, 10 months longer than prosecutors recommended. A transcript shows Zloch was silent on the point. Nor did Zloch say why he slapped down a defence request to shorten Birkenfeld's sentence and delay his jailing.

Nothing in Zloch's one-page order gives a reason for the ruling or its timing. He decided the matter without waiting to hear whether the prosecution opposed or supported the motion. What he might have heard was CBS's 60 Minutes report the night before his ruling, in which Birkenfeld said he shouldn't be going to prison at all.

Each side has a different view as to how heroic were Birkenfeld's efforts. To hear prosecutors tell it, for all the whistleblowing he did implicating UBS, he didn't immediately confess his own crimes and those of his richest client.

It's true Birkenfeld was deeply involved in the crimes he reported. As one of UBS's international bankers, it was his job to fly from his base in Switzerland to the US to persuade wealthy Americans to come to UBS, where they hid assets from the tax man.

If Birkenfeld had confessed his own sins from the first day, he probably wouldn't be facing prosecution at all, Downing told the judge in August. That is preposterous, says one of Birkenfeld's lawyers, Stephen Kohn. Birkenfeld went to every federal agency he could think of with his explosive information, with documents and with a willingness to go undercover if they wanted. The banker told investigators everything they wanted to know, says Kohn, who is also executive director of the National Whistleblowers Centre in Washington.

Prosecutors misrepresented that fact at the sentencing hearing, Kohn says, and he is seeking a justice department investigation of the handling of Birkenfeld's case. And yet, Birkenfeld's lawyers didn't object when Downing claimed in court that their client had been less than entirely candid. In fact, one of them conceded the point.

But because Birkenfeld brought such an historically important case to the feds, they asked Zloch to sentence him to five years' probation, including six months in home detention. Besides, even major tax evaders who admitted their crimes only when caught have not had to serve time.

Pick one side or the other, and you still can't explain why the judge ordered a sentence longer than prosecutors sought. Downing told Zloch at the hearing that he might later request a sentence reduction. If that's what he had planned, Zloch didn't give him the chance. Is it possible the judge acted that quickly because the television report the night before irked him?

And why did the judge assign more time than the obviously tough prosecutor wants? Why send a signal to those contemplating blowing the whistle that they would be better off keeping their mouths shut?

I called Zloch's office to ask about Birkenfeld. An assistant checked and said the judge declines to comment because it's an open case. At 66, the Reagan appointee to the South Florida court has a reputation as a tough judge. The week before he nixed Birkenfeld's latest effort, he suspended a Florida lawyer from practising for three-and-a-half years for accusing Zloch in the press and in legal briefs of harbouring a religion-based bias against his clients.

But at least he explained his decision in a 68-page ruling. The lawyer "has publicly impugned the dignity" of the judge and the entire court "with scurrilous and baseless accusations," Zloch wrote.

On second thought, perhaps Birkenfeld is lucky the judge didn't explain why he is going to prison.

Bloomberg