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The significance of any film (or novel or poem or for that matter theoretical work or even symphony or ballet) is determined by the reader/viewer/etc, not either the intentionof the maker _or_ the "facts" behind the work.
For the viewer of Waterfront, it was just The Union in general, not some particular corrupt union that came across. Most viewers would not know the difference between the ILWU, the ILA, or (for that matter, the IWW. It was a good movie, but like Yeats's "Irish Airman," utterly vile as well.
Carrol