It's not difficult to understand why Tim Donaghy, the rotten ref, is offering unsavory information about NBA officiating.

It's not difficult to understand why the media and the public are ready to give at least some credence to Donaghy's assertions.

What is difficult to understand is why NBA commissioner David Stern is so smug in the face of Donaghy's charges leveled against the league.

Stern immediately began playing D when Donaghy, who is awaiting sentencing for gambling on games in which he officiated, claimed that a 2002 playoff series was rigged by referees and officials, and that the NBA routinely encouraged referees to call bogus fouls to manipulate games while also discouraging them from calling technical fouls on star players to protect TV ratings.

Stern immediately labeled the charges as baseless and insisted Donaghy was the only rogue referee. "... the accusations that we manipulate games that then get reported on ... the facts underlying those, they're false," Stern said. "We don't."

It would be easy to dismiss out of hand anything a convicted felon says — except that the NBA didn't catch Donaghy in the first place, the feds did. So where does the commish get off being so confident in his system of policing his officials in the first place?

When Donaghy was busted last year, Stern said that there had been nothing suspicious about the frequency of Donaghy's foul calls, his bank account activity or anything else that would have tipped off the league. After boasting that the NBA's system of monitoring referees gives the league the best officials in sports, he said he wasn't surprised Donaghy went undetected.

"If you're intent upon engaging in criminal activity," he said at the time, "and if you are acting alone in many cases without the knowledge of even your family, it's possible. Our history is replete with examples of that. So it doesn't come as a surprise that you could go undetected."

But now he's saying he knows for certain that the league is clean.

According to Stern, Donaghy, an NBA referee for 13 years, was rated in the top tier of officials.

But Stern is sure the rest of the refs are good to go. Fool him once, shame on him; fool him twice — couldn't happen.

The commish is certain he has a handle on the integrity of NBA officiating, but he didn't know about the eight referees who were charged with tax fraud in 1998. He didn't know they were downgrading first-class airline tickets purchased by the league and pocketing the difference without reporting the income to the IRS.

But he's sure he knows the rest of the referees are clean.

He didn't know about veteran referee Joey Crawford's personal feud with Spurs' star Tim Duncan until the ref ejected Duncan from a 2007 game against Dallas and, according to Duncan, challenged him to a fight on the court. Crawford was suspended for the rest of the season.