No, it means if that you have a hobby (any hobby) that so preoccupies you that you must lock yourself in your garage getting stoned all night while you leave your house doors open, and don't check your kids for over 11 hours, even after alarm lights and open doors are noticed TWICE, and you are so indiscrete about your "hobby" that strangers like Westerfield know, (how did he just "happen" to be at Dad's when Brenda was having her "meeting" with that night's swingers? Hmmm?), then perhaps they should not be totally amazed when a wolf finds an openning to snatch their innocent and unguarded lamb.
It's called "contributory negligence", a well established legal concept where percentages of responsibility is routinely apportioned. For example, if parents have a picnic on the side of the freeway, and then leave their toddlers while they go off picking berries or making whoopy in the forest, they can't blame only the children or the driver if one of them is run over.
If it is determined that the couple contributed to the child's death through negligence then they should face the consequences. Sounds like it would be tough to prove, however.
Their daughter is dead; they may or may not have contributed in some manner. Your post seemed to suggest that they are faking their grief. I don't think this is the case.