Drug paraphernalia has a specific use (or at least, let's assume that for the sake of argument), but youtube-dl is more like a crowbar that has legal and illegal uses.

If I have a crowbar I can legally use it all day long for construction purposes. As soon as I'm caught breaking into a house with a crowbar, it's classified as burglar's tools. At no point is the hardware store or crowbar manufacturer liable for a burglary for selling me a crowbar.