Don't remember Jack the rugaru all that well, but I'm pretty sure Dean didn't believe Lenore wasn't killing humans. Once he did, he let her go. Correct? The question wasn't whether a vamp who doesn't kill people should be let go, but whether it was possible that a vamp was not killing people.

The witches in S7... hm. What was that story? Was that the one with the insta-daughter? I think I missed that one.

In any case, it's still quite standard on their part to let monsters go if they haven't killed people. It may not be precisely mandatory, but I don't think it's far from it.