Well - for ISPs it would be a very bad idea to add advertising to the (not their!) content. The internet isn't just youtube where another banner wouldn't matter but also some very critical applications for hospitals, brokers, companies, goverments... Most of these would react very badly if someone would inject some of their own bits.
Next problem: Just because someone is transfering a file which is copyrighted doesn't mean he is doeing it illegal - even is he is transfering to a swarm. Maybe he is the artist. Maybe this is promo material. Maybe a blockbuster hollywood film uses public domain material which the filter identifies.
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