With children, as I said earlier, part of the blame lies with their parents for not teaching them any better. I was about 4 when I understood the concept of what a stranger was, and my parents stressed to me the importance of not trusting strangers, even if they were in a familiar place, such as my school. I'd say the kid takes the whole blame at around 8 or 9, when they're definitely old enough to know better (or at least should). At this age I was required to walk to school, so my father taught me basic hold breaks and gave me a pocket knife for protection.
As far as the "level of abuse deserved", you get what you get; I don't think there's any way to gauge that. Some are more unfortunate than others.
I don't know enough about the Elizabeth Frizl case to give any sort of opinion on it, but I'll use a case I do know. Those girls that were found in Cleveland, Ohio earlier this year had plenty of opportunities to escape during their ten years of captivity (they said so themselves); after a while they weren't restrained in any kind of way. So I'd say after they blew their first few chances for escape that it was their own stupidity keeping them in that situation.
I never once condoned rape or abuse, and do think the people that do it are criminals and deserve to be punished. I don't see the abuser as the "deliverer of justice". That doesn't take away from my belief that if someone was stupid enough to be tricked into that situation they deserved it.