Mandatory “captive audience” meetings in which companies argue against unionization are illegal, the National Labor Relations Board ruled in a case involving Amazon.com Inc., prohibiting one of employers’ most potent weapons against labor organizing campaigns.

After a lengthy consideration, the National Labor Relations Board has ruled that Amazon’s “captive-audience meetings” are a violation of the National Labor Relations Act.

The NLRB said that Amazon managers solicited grievances and made implied promises to remedy them if workers rejected the union during mandatory meetings at the Staten Island wareh