In the U.S. the FCC (Federal Communications Commission) makes certain frequencies available to broadcasters for public use. When an end-user pays to use that spectrum, jamming the signal is paramount to 'property theft.' The FCC is also concerned about potential "leakage" — of jammers interfering with frequencies outside the range of cell phones, like garage door openers or medical equipment; and it's worth noting that over 100,000 emergency calls are made each day from cell phones. Anyone caught manufacturing, selling, owning, or using a jammer in the U.S. is punishable by an $11,000 fine and up to a year in prison for each offense.
But the stiff penalty hasn't stopped proliferation of the devices, perhaps because the FCC has not held anyone accountable. According to one interview with Richard Welch, associate chief of the FCC's Enforcement Bureau, no actions have been taken by the FCC because "nobody has complained." This isn't surprising considering people can't tell the difference between being jammed and simply having poor signal strength which comes and goes with the best of phones even under normal circumstances.