Joe Nocera’s column in the Saturday Times is an excellent explanation of how sick the business practices were at AIG during the bubble.

First, the setup doesn’t mince words:

More than even Citi or Merrill, A.I.G. is ground zero for the practices that led the financial system to ruin.

“They were the worst of them all,” said Frank Partnoy, a law professor at the University of San Diego and a derivatives expert. Mr. Vickrey of Gradient Analytics said, “It was extreme hubris, fueled by greed.” Other firms used many of the same shady techniques as A.I.G., but none did them on such a broad scale and with such utter recklessness. And yet — and this is the part that should make your blood boil — the company is being kept alive precisely because it behaved so badly.

Nocera then calls AIG’s process of using its Triple-A rating to mint money with credit-default swaps a “scam.” He’s right.

Unlike many of the Wall Street investment banks, A.I.G. didn’t specialize in pooling subprime mortgages into securities. Instead, it sold credit-default swaps.

These exotic instruments acted as a form of insurance for the securities. In effect, A.I.G. was saying if, by some remote chance (ha!) those mortgage-backed securities suffered losses, the company would be on the hook for the losses. And because A.I.G. had that AAA rating, when it sprinkled its holy water over those mortgage-backed securities, suddenly they had AAA ratings too.

It’s crucial to understand why this was so (temporarily) profitable: Wall Street wanted to unload its risk from the crap mortgage securities it was creating, and AIG was the trash receptacle.

The regulatory arbitrage was even seamier. A huge part of the company’s credit-default swap business was devised, quite simply, to allow banks to make their balance sheets look safer than they really were. Under a misguided set of international rules that took hold toward the end of the 1990s, banks were allowed use their own internal risk measurements to set their capital requirements. The less risky the assets, obviously, the lower the regulatory capital requirement.

How did banks get their risk measures low? It certainly wasn’t by owning less risky assets. Instead, they simply bought A.I.G.’s credit-default swaps. The swaps meant that the risk of loss was transferred to A.I.G., and the collateral triggers made the bank portfolios look absolutely risk-free. Which meant minimal capital requirements, which the banks all wanted so they could increase their leverage and buy yet more “risk-free” assets. This practice became especially rampant in Europe. That lack of capital is one of the reasons the European banks have been in such trouble since the crisis began.

The problem for the financial system is that since credit-default swaps are unregulated (thanks, Phil Gramm, Larry Summers, and Robert Rubin!), AIG didn’t have to put any collateral in reserve, even though CDS are effectively insurance policies. That’s why the government had to bail out AIG. If it didn’t, all those banks who bought insurance from AIG, hedging their exposure to junk mortgages, would have to take those losses.

Here’s what is most infuriating: Here we are now, fully aware of how these scams worked. Yet for all practical purposes, the government has to keep them going. Indeed, that may be the single most important reason it can’t let A.I.G. fail. If the company defaulted, hundreds of billions of dollars’ worth of credit-default swaps would “blow up,” and all those European banks whose toxic assets are supposedly insured by A.I.G. would suddenly be sitting on immense losses. Their already shaky capital structures would be destroyed. A.I.G. helped create the illusion of regulatory capital with its swaps, and now the government has to actually back up those contracts with taxpayer money to keep the banks from collapsing. It would be funny if it weren’t so awful.

This column is just excellent explanatory journalism. It helps us to the “aha!” moment that enables us to see and understand the Wall Street sausage machine better.

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Ryan Chittum is a former Wall Street Journal reporter, and deputy editor of The Audit, CJR's business section. If you see notable business journalism, give him a heads-up at rc2538@columbia.edu. Follow him on Twitter at @ryanchittum.