[emn13]

First if all: merely finding some instances of copyright violation that reduce a copyrights value does not validate calling it theft. By that line of reasoning half of the English language is approximately theft. This very answer is stealing your copyrighted answer by undermining its validity, thereby reducing its value, therefore it's equivalent the theft.

That's a preposterously loose requirement. Secondly, even in your example, the damage isn't primarily caused by the copying, it's primarily caused by the misrepresentation. If the copier had been truthful about the source, it's doubtful the original would have been much devalued.

In any case, clearly copyright violation can devalue the copyright. It's certainly not always the case (it may even increase the value of the copyright), but it's a serious risk. However, that still doesn't make it equivalent to theft, any more than vandalism, fraud, competition, criticism, libel, etc. are theft. Some of those are even considered virtuous - as a society we supposedly encourage criticism and competition (even if many sectors manage to pay only lip service to that notion).