It’s hard to say what’s more exasperating: the woeful performance of the credit ratings agencies during the recent mortgage securities boom or the failure to hold them accountable in the bust that followed. Not that Congress hasn’t tried, mind you. The Dodd-Frank financial reform law, enacted last year, imposed the same legal liabilities on Moody’s, Standard & Poor’s and other credit raters that have long applied to legal and accounting firms that attest to statements made in securities prospectuses. Investors cheered the legislation, which subjected the ratings agencies to what is known as expert liability under the securities laws. But since Dodd-Frank passed, Congress’s noble attempt to protect investors from misconduct by ratings agencies has been thwarted by, of all things, the Securities & Exchange Commission. The S.E.C., which calls itself “the investor’s advocate,” is quietly allowing the raters to escape this accountability.