Why isn’t New York Attorney General Eliot Spitzer, who hasn’t shown any signs of backing away from a fight so far in his one-man crusade to reform corporate America, going after Warren Buffett with the same relish he seems to have pursued Maurice “Hank” Greenberg?

The difference in Spitzer’s attitude to the two is fascinating and puzzling, given that Buffett’s company, Berkshire Hathaway Inc., was on the other side of the transaction that cost Greenberg his job as chairman and chief executive officer of American International Group Inc., the world’s largest insurer.

Spitzer calls Buffett a “cooperative witness” in his investigation into the insurance industry. Greenberg, by contrast, may suffer an “adverse impact” by invoking the Fifth Amendment to avoid testifying, Spitzer says.

AIG paid a $126 million fine in November to settle allegations it helped clients such as Pittsburgh-based bank PNC Financial Services Group Inc. remove bad loans from its books. Greenberg resigned March 14 because of the investigation into whether AIG used a similar shuffle to polish its own earnings in a contract with General Re Corp. – a unit of Berkshire Hathaway.

AIG’s use of reinsurance contracts may have helped inflate its net worth by as much as $1.7 billion over 14 years, the company said on March 30. The General Re transaction added $500 million to AIG’s premiums and reserves for claims.

While Buffett “is not a subject or a target of our investigation,” Greenberg is “a CEO who did not tell the public the truth” and might face a civil or criminal case, Spitzer said on ABC television on April 10. Buffett was interviewed by regulators the following day.

Buffett would be a formidable adversary even for Spitzer, who plans to run for the New York governor’s office in elections next year. A poll of voters published Feb. 10 showed he’d beat the current chief, George Pataki, by 54% to 30% if Pataki sought a fourth term.

Would Spitzer risk hurting his ratings by pursuing the Sage of Omaha? Buffett has been an outspoken critic of corporate malfeasance with a reputation for living a humble, simple existence characterized by straight talk and cheeseburgers. He holds a special place in the affections of U.S. retail investors, who dream of replicating his investing acumen.

Contrast the following portraits of the two insurance executives, supplied by the Wall Street Journal in separate articles earlier this month:

“Mr. Buffett answered questions for regulators and prosecutors, sipping a Coke, and interspersing his answers with tidbits about his management philosophy in folksy anecdotes, according to people who were at the interview.”

“In his executive suite filled with Chinese artifacts, Mr. Greenberg had his own elevator guarded by his own security detail, his own living room adjoining his office and private chandeliered dining room. At certain times, when AIG executives traveled with him on business, they were required to use the small pilots’ bathroom in the front of a corporate plane. A large, fancy bathroom in the back of the plane could be used only by Mr. Greenberg, his wife and their Maltese dog, Snowball, according to a former AIG executive.”

AIG and General Re used something called finite-risk reinsurance, which isn’t illegal. When it’s used as a disguised loan to mask losses, inflate capital, or boost earnings, however, regulators start to take an interest.

AIG has since said the transaction shouldn’t have been classified as reinsurance because it involved no risk to General Re. The U.S. Financial Accounting Standards Board said this week it may tighten rules on the use of such contracts in the next six months.

“Mr. Buffett was not briefed on how the transactions were to be structured or on any improper use or purpose of the transactions,” Berkshire Hathaway said in a March 29 statement.

Not only has Greenberg lost the stewardship of a company he spent four decades building, regulatory filings this week showed he transferred $2.2 billion shares of AIG, or 96% of his direct ownership stake, to his wife, Corinne, four days before his resignation. You only do that if your lawyers have warned that your financial assets might be threatened by lawsuits.

Two insurance chiefs, two insurance companies, each in the business of something called finite reinsurance, which may or may not be more about accounting sorcery than redistributing risks. So far, though, two very different outcomes for the executives involved.

Mr. Gilbert is a columnist for Bloomberg News.