This is awfully judgemental, when we don't really know all the circumstances or the extent/nature of Tina's mental disability. Suppose you were mildly retarded, and a woman who's a relative of your husband and has been babysitting your infant convinces you that the state is going to take your baby away from you and never let you see her again, but if you give her the baby and tell police she just "disappeared", the woman says she'll hide the baby until things calm down and then let you see the baby whenever you want. So you're scared of losing your baby forever and decide you'd better go along with this plan. Then once the woman has your baby, she warns you that if you tell police what really happened, she'll kill the baby and hide the body so there won't be any evidence (against the woman).
Tina may very well have been faced with some heavy manipulation, deliberately designed to frighten and confuse her, that she simply didn't have the ability to figure out and react appropriately to. The accusations that Tina is fully responsible for this strike me as every bit as inappropriate as when many people (not here, as far as I know) were claiming Elizabeth Smart obviously went and stayed willingly with her kidnapper -- it was just so "obvious" that she had many opportunities to escape and "chose" not to. I never bought into that view, but many peopel did, and just recently we finally heard a lot of the details, including that she was forcibly drugged and drunk much of the time. Objectively speaking, sure, there were many times when she was out in public with Brian and Wanda, and could have pulled down the veil and yelled "Help! I'm Elizabeth Smart! I need to get away from these people!" Seems easy, but it just wasn't for her. And remember, Elizabeth was a mentally normal (in fact, very smart, it seems) 14-15 year old during her ordeal. Tina has been reported by a family member to have a mental age of about 13.
If we're going to call for punishment of anyone other than SB (and JB if he knew what was going on), it should be the social workers who had plenty of opportunities to interview Tina, and to request evaluations from psychologists/psychiatrists, and who had been advised by Tina's mother that Tina wasn't capable of parenting -- and yet still insisted on leaving Tina and Rusty (who has also been reported to have some mental deficiency), raising Shannon independently.