AP Photo

CASABLANCA—For almost a year, diplomats, experts and journalists have been telling investors not to worry about the new Islamist governments emerging in the wake of the Arab Spring. They are wrong.

The optimist argument was simple: “democratic realities will force Islamists to maintain economic growth in order to win the next elections.” Arab spring will not immediately shift to winter; responsibility makes politicians responsible and so on.

Certainly, the Islamists talked a good game. Morocco’s Islamists, in their party platform and in their speeches, stressed that a freer economy would attract foreign investors and boost jobs. They said that reducing youth unemployment was their No. 1 issue. They said that they wanted to return annual GDP...