As it was pointed out the first time youtube-dl was on Hacker News, our user base seems to be split regarding that behavior. When you copy a URL from a playlist, the URL includes both the video id and the playlist id.

Some people want to download the whole playlist when passing a URL from one of the videos (because otherwise it's hard to get an URL that contains just the playlist). Some other people just want to download the video they are currently watching.

That's why we have the --no-playlist option, which you can use and even stick in the configuration file. There's also --yes-playlist to override the configuration file if you need to.