Ok, first of all, no, these are not the rebels that the US government wants to back that are doing this.
The other problem is that everyone ignores the words 'hardline salfists'. Salafi are to the sunni what the KKK is to white protestant Christians. They espouse a whole lot of ideas that even their fellow conservative Muslims find abhorrent.
Third: according to Tunisian papers interviewing these women (and they are indeed, women, not young girls) these women are going in the name of religion.
http://www.alhadathnews.net/archives/101039
Lamia became convinced that a woman may participate in the jihad to eliminate the enemies of Islam by making her body recreational for the men after each and every raid, so that her body became their possession.