>> By and large, it ought to be possible to get copyright filtering for entire works to work at least as well as spam filtering
In spam filtering you have an adversarial relationship between the sender on one side and the recipient+carrier on the other side.
With copyright filtering the sender and receiver are both on the same side, with the carrier being a (probably somewhat unenthusiastic) adversary trying to thwart the transmission. The real adversary is a rightsholder who really doesn't have a direct role in the transmission use case.
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In spam filtering you have an adversarial relationship between the sender on one side and the recipient+carrier on the other side.
With copyright filtering the sender and receiver are both on the same side, with the carrier being a (probably somewhat unenthusiastic) adversary trying to thwart the transmission. The real adversary is a rightsholder who really doesn't have a direct role in the transmission use case.