Moreover, that would save the law from being a dead letter in the sense that there would be circumstances when it could theoretically be enforced (i.e. when someone is overtly marketing it as a tool for infringement), but wouldn't anybody making the tool then just not do that? It's still exactly the same tool. And then what does the tool have to do with anything anyway, when what you really have is a law against speech promoting copyright infringement?
HN is not a court of law, most people here are not lawyers, let alone well-paid and competent lawyers like the ones the RIAA can afford.
> wouldn't anybody making the tool then just not do that?
Sure - which is why you're not seeing many ads for tools to steal cars or pick locks. But it's not only about the way the material is promoted, it's also about how it's obtained, whether the seller could reasonably know what it would be used for, and a bunch of other stuff.
You can't look at this from a binary perspective; despite depictions in popular media, the law is typically not about "aha" moments, but rather about putting a number of coherent pieces together to paint a certain picture.
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As a matter of policy (as well as legality), youtube-dl does not include support for services that specialize in infringing copyright. As a rule of thumb, if you cannot easily find a video that the service is quite obviously allowed to distribute (i.e. that has been uploaded by the creator, the creator's distributor, or is published under a free license), the service is probably unfit for inclusion to youtube-dl.
A note on the service that they don't host the infringing content, but just link to those who do, is evidence that the service should not be included into youtube-dl. The same goes for any DMCA note when the whole front page of the service is filled with videos they are not allowed to distribute. A "fair use" note is equally unconvincing if the service shows copyright-protected videos in full without authorization.
Support requests for services that do purchase the rights to distribute their content are perfectly fine though. If in doubt, you can simply include a source that mentions the legitimate purchase of content.
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A tool designed to be used by police and marketed towards police is legal, as its intended purpose is not infringement. A tool advertised as "hey, kids, use this to infringe those copyrights" is totally different, even if it's literally the same tool.