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About John Cage's infamous concept piece 4'33" and his equally infamously litigious estate:

They claim copyright over any "recording" of silence of that length, including any purported sample thereof.

However, my understanding is that the silence of the performers is only a backdrop for the substance of the piece.
It's about the sounds the audience makes.

Without the sounds of the audience, it's not a performance of 4'33".

Has this seriously not been brought up in court, to invalidate these claims?