> Perhaps a lawyer can explain a little more to me the logic behind the “transformative” rule when it comes to copyright protection
IANAL, but…
First, its not a rule of copyright protection, its a rule of fair use, which is only an issue when copyright protection exists.
Second, there is not really a specific “transformative” rule. Whether use is transformative is an aspect of the “Purpose and character of use” factor in fair use analysis. That a use is “transformative” (that is, that it has a different character and use – that it becomes a work of a significantly different kind) is typically, where present, a finding which ways in favor of fair use, but it is neither by itself neither necessarily necessary for nor necessarily sufficient for fair use (though in a specific case it can be either), there is no hard and fast rule, and fair use analysis is highly context-specific.
> So basically what I’m asking is why do judges appear to apply the “transformative” rule to visual arts but not to auditory ones.
The defendants in the Blurred Lines case did not assert fair use, they asserted that there was no use at all, and that there was no copying and no substantial similarity in the works resulting from copying. Whether they might have prevailed had they done so instead of denying copying at all is…speculation, but it is not an inconsistency that they did not win on an argument they did not make, even if consistency would demand that they should have if they did.