I do understand the confusion. In case one has an instability of the back it does not make a lot of sense to stretch the hamstrings since this might be the last protection to avoid more backproblems. One can think that it might be the last restraint to avoid a total colapse. But the decreased mobility of the hamstrings might result in more movement of the vertebrae suffering from Wikipedia reference-linkspondylolisthesis
In order to restore stability of the back it seems to me that the first resort should be increasing core stability. Segmental as well as global. When you do this you might find that the hamstrings respond with a lower tone and an increased mobility. Research has shown that stretching any how is questionable and that it does not have any longterm effect.