At the risk of a little pop-psych thinking, I strongly suspect that the "blame the parents" crowd, on this and similar threads, make their comments as a way of assuring themselves that THEIR kids (grandkids, nieces, nephews, whatever) would never find themselves in this situation. It's in large part a defensive reaction to horrific news -- a way to reassure yourself that you and yours are not in danger.
What a brilliant analysis! (I was thinking something along those very lines myself yesterday). :-)
You know the problem with pop-psychology? It's a two-edged sword. Someone could say (with just as much evidence and legitimacy as your original "analysis") that the "let teenagers be teenagers" crowd are making their statements to unconsciously reassure themselves that whatever bad actions or experiences their own children engaged in does not reflect on their parenting or moral duties. They could be attempting to rationalize their own feelings of parental failure and their secret, guilty feelings that they did not do "enough" to prevent their own children from straying.
Which is why pop-psychology is garbage. Do everyone a favor, and avoid that kind of psuedo-scientific BS. Analyzing and arguing from someone else's motives (rather than their arguments) is a logical fallacy, and not productive. If you think the linking of these childrens' situation and the parents' responsibilities is in error, then attack that (and there's a lot to attack). But attacking the "motive" of the arguers (ad hominem) is a sign of intellectual weakness, not strength.
If I wanted emotional reaction and fallacious argument, I'd read DU. We all expect better from Free Republic...