I followed the Napster trial closely. Napster’s argument that it had legitimate use was demolished because it had an ability to monitor the specific use of its

I followed the Napster trial closely. Napster’s argument that it had legitimate use was demolished because it had an ability to monitor the specific use of its service on an ongoing basis. That is, it could see queries from users as they happened. In fact, this ability was at the core of the service since Napster relied on a centralized search index maintained by the company. The court ruled this gave the company a more active role in contributing to infringement than the precedent Napster was trying to use (which involved VCRs - the Betamax case). The courts remedy (at least at the preliminary injunction phase) was not to shut down Napster but to force it to comply with search blacklists provided by the record companies. youtube-dl obviously is not a service so had no ability to monitor ongoing use. IMO it would have a much stronger case for legitimate use. IANAL