Guess all those metal buildings in the US would be illegal since they block access to wireless networks.

Hell take a wire and stick it in the high level output of an older laserdisk player, run it around the room and you got yourself a jammer. The first generation of those things would drown out an entire spectrum.

Under current rules: intentional jamming is illegal, so is unintentional (unless the interfering device is operating as it is supposed too) but it's tough to enforce since it takes some equipment to find it and prove it.

Today's modern cell phone REQUIRE close proximity to a cell station to function properly since they are so tiny. Today .025 of watt is huge, 10 years ago they operated at 5 watts.

Regardles of Active or Passive blocking the outcome is the same, you walk into a building and contact with the cell controller is lost.

\\EDIT Besides... according to the article private use of Cell phone blockers is illegal in the USA.