I think the answer as to "why" women would do this is multi-faceted. My guess is that these women most likely have extremely low self-esteem due to early abuse/loss/neglect issues which have never been treated or resolved.
They might have had children thinking that this would fill the void in their life, only to then resent the children as being too needy, too much work. The circle just goes round and round. Not being tremendously bonded nor imprinted with the protective sense most mothers have, they see their children as "problems" and "nuisances". They get in the way of the mothers attempting to have any sort of fun--partying with a new boyfriend, going out drinking with girlfriends.
They leave the children to fend for themselves and the children get into more and more trouble and develop greater behavior problems as they have no guidance, no supervision, no solid love. They are passed off to family and friends for care--some wonderful, some not so great, some downright awful. The mothers are locked in their own little world of "me vs. them".
When a sick guy comes along and asks them to do something horrible like this for money or for favor, I don't think it's a huge stretch for them. They might say they love their children, that they'd do anything for them. They might actually believe that as they go to work and bring home a paycheck and curtail some of their partying for the kids. They buy Christmas presents and every once in a while have a fun day with them. They may set up Myspace pages full of photos of smiling children. But the whole relationship is hollow and has no foundation. With no foundation, everything comes tumbling down. JMO
See where I'm going? We MUST recognize those little girls today who are born into less than wonderful homes, have no support systems, aren't making healthy societal connections or choices, aren't getting therapy for their own hurts and abuse. The little girls and teens who are empty. If we don't, we will have more and more cases like this to look forward to. Abuse and neglect have a multi-generational impact. It's a strong and committed soul who steps away from a childhood such as I described and becomes a loving, caring, and responsive parent. It can be done but it's an uphill battle.
Am I advocating to let these women off easy as they might have been abused? Heck no!! But I'm trying to make some sense out of how they got to point where they'd abuse their own child for a perverted man's pleasure.