On one covert video, farm workers illegally burn the ankles of Tennessee walking horses with chemicals. Another captures workers in Wyoming punching and kicking pigs and flinging piglets into the air. And at one of the country’s largest egg suppliers, a video shows hens caged alongside rotting bird corpses, while workers burn and snap off the beaks of young chicks.

Each video – all shot in the last two years by undercover animal rights activists – drew a swift response: Federal prosecutors in Tennessee charged the horse trainer and other workers, who have pleaded guilty, with violating the Horse Protection Act. Local authorities in Wyoming charged nine farm employees with cruelty to animals. And the egg supplier, which operates in Iowa and other states, lost one of its biggest customers, McDonald’s, which said the video played a part in its decision.

But a dozen or so state legislatures have had a different reaction: They proposed or enacted bills that would make it illegal to covertly videotape livestock farms, or apply for a job at one without disclosing ties to animal rights groups. They have also drafted measures to require such videos to be given to the authorities almost immediately, which activists say […]