If a narah me’urosah is violated in the field her assailant is put to death but she is not punished for he did so against her will

The witnesses were on the other side of the river. (Today, I would wonder if “hasrah” can be done through video and speakers). (Sheilah Yunger)

Another answer, which the Moshav Z’keinim suggests, is that the witnesses came in the middle of the sin, which was sufficient for the purposes of warning and testifying against the man but too late to save the woman.