Musical plagiarism can be hard to explain for non-musicians. Because of the ubiquity of certain chord progressions, there are things that sound like copying but are really more like convergent evolution. (Like, bats didn’t copy their wings from birds…)
A distant cousin, Rob Mathes, recently served as an expert witness defending Led Zeppelin in a plagiarism case concerning “Stairway to Heaven.” His account of the case, and the intricacies of what is copying, musically speaking, is fascinating. Suffice it to say, Led Zeppelin didn’t plagiarize the piece, and the musical similarities are natural for pieces born of the same era, and both of which reference a long musical lineage.
(I think the same is true for “Blurred Lines”, though Pharrell Williams and the rest didn’t have musical experts as proficient as Rob, apparently.)
Plagiarism in writing and speech…that’s a different story. We can all recognize when a speech has been copied.