One of the dangers of making any link in the internet chain pre-emptively responsible for any kind of enforcement is that that link then becomes legally responsible for that enforcement and can be sued (or worse) if any non-enforcement takes place.
Basically, if the ISPs become responsible for detecting unauthorised content, they become financially culpable for any infringement that occurs through their systems.
And how is an ISP supposed to discover the infringement? They are just passing the bytes through.
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