So you'd rather let the daughter set her boundaries in fear of her leaving? At age 15? It sounds like that brings up some major control issues.
I'm not trying to bag on anybodies way of raising their children (albeit, maybe the people mentioned in this news articles'). I don't have any children, and am hopefully still a pretty good ways away from being there. But I felt like my parents did a pretty decent job of raising me into a responsible adult.
Yeah, they gave me leeway and let me do things they probably didn't want me doing. But they also know when to draw lines and make me understand what was wrong, even if I didn't like them for it. That seems like the double-edged sword of parental responsibility, but when it comes down to it, your parents will always be your parents and your children, children.
The fact that these parents had given up on a 15-year-old girl and let her move in with a guy who, from the sounds of it, has a history of physical abuse, for any period of time is a testament to the fact that they obviously weren't trying very hard.