It is my understanding that while polygamy is technically legal in many Arab countries, it is not common and only a very small percentage of marriages are polygamous. Since there does seem to be a widespread problem with the treatment of women and children, there must be an explanation beyond polygamy since the vast majority of this ill-treatment is occuring in non-polygamous households.
Just because all unions are not polygamous doesn't mean that the vicious treatment of women in polygamous societies is excused.