Why does the SEC protect banks’ dirty secrets?

From Bloomberg:

Richard Bowen … is the former senior executive at Citigroup Inc. (C) who in November 2007 issued a clarion call to his colleagues and Citi’s board that a major credit-quality problem loomed for the bank.
What he had discovered was that 60 percent of the home mortgages that Citigroup had bought from third parties, or $30 billion, were “defective,” meaning that they didn’t meet Citigroup’s underwriting criteria. Nevertheless, they were still packaged up — defects and all — and sold as securities.
You know where this is going. The Citigroup executives did next to nothing.