This year's Berkshire Hathaway AGM, usually known as the "Woodstock for capitalists", did not pass without some uncomfortable moments.

Like when Warren Buffett was taken to task over his ethical standards. One of the issues was Berkshire Hathaway's stake in PetroChina, a subsidiary of China's state oil company, CNPC, which has extensive interests in Sudan and which, according to activists, is linked to the Darfur genocide.

The Guardian reports Jason Miller of the Sudan Divestment Taskforce claiming that CNPC is "the most irresponsible and abusive oil investor in Sudan."

"At least 70 per cent of the funds they provide to Sudan get funnelled into the Sudanese military," said Miller.

The resolution didn't get up, which was to be expected.

But there are important questions to be asked, says Tom Barlow at the Bloggingstocks site.

"A complication to the issue might arise as Buffett makes good on his promise to donate most of his Berkshire Hathaway stock to the Gates Foundation. Given the Foundation's strong involvement in Africa and with the poor, as it begins to accumulate BH stock I'd expect it to ask hard questions about the PetroChina holdings. Ironically, it could be from this board that renewed pressure will be brought on the company to divest itself of PetroChina."

Still, for his part Buffett defended the conduct of corporations, casting doubt on efforts to rate the performance of companies on social, human rights or environmental measures and claiming he wouldn't consider such factors when selecting investments. And that includes tobacco.

"I don't know how I would rate Exxon versus Chevron versus BP," said Buffett, reports the Los Angeles Times. "It's very difficult to judge the actions of companies that act on thousands of things every day."

Significantly, a Times investigation found that 87 per cent of Berkshire's publicly disclosed stock holdings had been invested in companies criticized for slack environmental practices and human rights abuses.