Because if it's just querying for metadata that anyone can already query for...your point seems immaterial as to the legality of the tool?
I've been in several situations where this would have been incredibly handy, but never realized it was possible.
There's all kinds of cool stuff you can do with youtube-dl. For example 'ytsearch20:kittens' will get a playlist of the first 20 search results for 'kittens'.