I would think it has little to do with the benefit of the doubt and everything to do with retaliation and escalation. If the South found that the North definit

I would think it has little to do with the benefit of the doubt and everything to do with retaliation and escalation. If the South found that the North definitely sunk a South submarine, the people would (more loudly) call for retaliation. And while everyone is certain what that would lead to, no-one's quite sure where it would end and no-one doubts that the losses for all involved will massively dwarf the loss of a single ship. And there's always the possibility that the South really was in the wrong. That their ship had violated an agreement by doing something they'd rather not publicize. (Not unlike the subdued US reaction to the EP3/Mig-21 collision in 2001. Were the US 'innocent' the rhetoric would have sounded much different than it did. But even then, the risks of escalation far outweigh the losses at hand.)