Excerpt from this article:
A Saudi scholar issued a fatwa against using another person’s WiFi without permission, since theft cannot be tolerated in Islam.
…The WiFi fatwa is far less provocative, although the need for it confused some commentators. Why not just encourage people to put passwords on their private Internet?
“We do not need a religious edict to pinpoint such basic things,” a Saudi blogger noted, according to the Gulf Times. “Private property should remain private, especially [since] the owner paid money for the services. Nobody should just take advantage.”