The Congressional Budget Office has lopped $20 billion off its estimate of the cost of keeping Fannie Mae and Freddie Mac afloat for the next decade—to a mere $79 billion. That will have to pass for good news, even if the estimate comes loaded with caveats. The bigger story is why the White House continues to keep these wards of the state off-budget.

As the CBO notes in a recent background paper, the standards for when to include government-sponsored entities in the budget go back to the 1960s, when a Presidential commission...