Many GSE watchers were surprised by the Justice Department’s recent decision to conclude its nearly two-year investigation of Fannie Mae without bringing criminal charges against the company. But GSE observers and legal sources suggest last year’s unanimous U.S. Supreme Court ruling involving the government’s post-Enron pursuit of Arthur Anderson likely played a decisive role in Justice’s decision to close its probe. Last May, in a 9-0 decision, the nation’s highest court threw out the conviction of the accounting firm...