I personally don't think people who are into learning things about permaculture and sustainable farming/homesteading/primitive building etc, have wicked intentions, and I don't think that this type of farmers/hosts have those intentions either.
But I can imagine many people who go "off-grid" or people who are attracted to that lifestyle might have had "troubled pasts", but that on the other hand doesn't necessarily make them bad people.
I agree with the first statement, but our experience has been there are those who are looking for a free place to stay and don't want to learn or contribute. Those are the ones we seek to screen out. It's happened a few times to us, always when the references weren't great.
Troubled pasts don't bother us so much as not having good references for who you are now.
Yeah, I understand you don't want some lazy "zombies" lurking around on a farm... That must be horrible... =)