Music piracy groups mostly upload copies they have purchased or otherwise legally gained access to. Their torrents are a public good since they serve as a benevolent institution for those that are very disadvantaged and might not have the money to buy the content otherwise, allowing them to participate in modern culture.

For almost any kind of digital "piracy" you can make arguments that sound very similar to yours, because in the end libraries and piracy do something very similar: both buy content once and redistribute it for free. The only reason we treat libraries different is because of history and because libraries have a bunch of physical restrictions that seem silly in the digital world.