In the United States, United Kingdom, Australia and many other countries, blocking cell-phone services (as well as any other electronic transmissions) with a mobile phone jammer is against the law. In the United States, cell-phone jamming is covered under the Communications Act of 1934, which prohibits people from "willfully or maliciously interfering with the radio communications of any station licensed or authorized" to operate. In fact, the "manufacture, importation, sale or offer for sale, including advertising, of devices designed to block or jam wireless transmissions is prohibited" as well.
Jamming is seen as property theft, because a private company has purchased the rights to the radio spectrum, and jamming the spectrum is akin to stealing the property the company has purchased. It also represents a safety hazard because mobile phone signal jammer blocks all calls in the area, not just the annoying ones. Jamming a signal could block the call of a babysitter frantically trying to contact a parent or someone trying to call for an ambulance.