Federal law on unions and organizing applies to American Indian-owned casinos on tribal land, a federal appeals court ruled April 26.

The decision upholds a 2013 ruling by the National Labor Relations Board, which found that it that it had jurisdiction over an organizing drive the union UNITE HERE conducted at Casino Pauma. The board is the federal agency that administers labor law for most employers.

The National Labor Relations Act is ambiguous with respect to its applicability to tribal governments, Judge Marsha S. Berzon wrote for the U.S. Court of Appeals for the Ninth Circuit. The NLRB’s determination should...