The nation's megabanks and Republicans regularly rail against re-regulation of the banking industry to rein in practices that spawned the 2008 financial implosion that haunts the economy now, and that will continue to haunt it for years to come. But they have nothing to say when evidence of Wall Street's casino-style gambling on toxic securities, and outright duping of their own investors, surfaces and proves again the need for the tighter rules scheduled under the Dodd-Frank Act.

Take, for example, the settlement last week by Citigroup, one of Wall Street's biggest banks. It admitted in a civil complaint brought by the Securities and Exchange Commission that it had led some of its investors to buy shares in a derivatives package of toxic mortgages that the bank itself had developed -- and had then bet against the package. When the portfolio containing the bank's hand-picked toxic mortgages failed, the investors lost hundreds of millions of dollars, while the bank profited handsomely from its secret hedge bet against its own portfolio.

The SEC's punishment? Citigroup agreed to pay a penalty of $285 million. That's sounds like a lot, but it's a mere slap on the wrist relative to the amounts involved in the scam and the duping of investors in the $1 billion portfolio. It barely dents the bank's profit margin. Citigroup last week reported a third quarter profit of $3.8 billion on revenue of $20.8 billion. Clearing its books of the SEC complaint was apparently worth it to the bank.

Part of the fine will be distributed to investors in the tainted portfolio, known as Class V Funding III. The bank had earned $126 million in its bets against the portfolio, and another $34 million for assembling it. Those amounts are part of the settlement funds to be returned to the duped investors.

Most people would describe such financial flim-flam as an outright fraud, yet there's been no criminal action for fraud against Citigroup -- or, in previous similar cases against Goldman Sachs and JPMorgan Chase & Co. But as the Occupy Wall Street protesters have come to understand, this is how business has been done on Wall Street.

The SEC. simply issued a statement saying that "securities laws demand that investors receive more care and candor than Citigroup provided" to its investors in the derivatives portfolio. "Investors were not informed that Citigroup had decided to bet against them and had helped to choose the assets that would determine who won or lost," said Robert Khuzami, the SEC's enforcement division director.

Citigroup's self-serving statement simply noted that the S.E.C. did not charge it with "intentional or reckless misconduct." Oh, yeah?

You have to put that disclaimer in this context: This wasn't Citigroup's first stumble related to the financial crisis. It agreed last year to pay $75 million (more chump change) to settle other federal complaints that it failed to inform investors that it held a huge chunk of subprime mortgages which not only were losing value, but which finally forced the government to rescue it with $45 billion from $700 billion TARP funds.

Small wonder Wall Street's mega-millionaire masters of the global financial universe, whose colleagues regularly rotate at the helm of the Treasury Department and in Washington's elite financial councils (think Summers, Paulson, Geithner), feel privileged to walk away free of criminal fraud cases for such egregious and secret financial plots.

All this seems absurd, of course, but it's on par with the lax enforcement that has led to the Dodd-Frank Act. So why, precisely, are Republicans still trying to kill banking reform?