Iran is pardoning them and setting them free. That way the UK can continue to claim that Iran was wrong and Iran can continue to claim the UK was wrong.

The Iranian's probably looked at the situation and decided they had nothing further to gain from holding the soldiers. Insofar as they where violating a contested border, Iran had made it's point. And holding the soldiers further would make them look bad, nor was their any more propaganda to be gained from showing them on television.

Jay