A society that doesn't allow math won't last long.
They do allow math. Just not for everyone. They keep children stupid, except the ones that work for them using and creating technology to keep them in power. The leaders of the group know that Islam - and religion in general - is bull shit, but - as in our countries - religion keeps people dumb and easy to manipulate. As Dostoevski wrote in his parable about the cardinal great-inquisitor.