This is the core of the issue.
Politically, releasing Jaime was akin to suicide and/or death for Robb's army. As a mother of a dwindling family, the move is understandable. Even then, however, as the mother of the King of the North, who is currently engaged in a war with a family who is known to be less than trustworthy, Catelyn done smurfed up.
Catelyn's move, while understandable in certain aspects, was one of the reasons why she is one of, if not the, dumbest characters in the books and series. Kudos to the actress for the great portrayal.