The 62-year-old rocker-activist says he was told a local faction of the International Alliance of Theatrical Stage Employees union planned to picket the show.
Young and his wife are honorary lifetime members of the union, which is striking against The Forum's owner, Faithful Central Bible Church, because of contract disputes.
Young says he is "extremely disappointed to have to choose between satisfying my fans or backing my brothers and sisters of the IATSE."
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A Los Angeles date will be rescheduled in 2009, and refunds for tonight's show will be available at place of purchase.
Young's hits include 1989's "Rockin' in the Free World."