Ask HN: Why youtube explicitly doesn't allow downloading videos?

Why does youtube does not have any "download" button for it's videos. After all these videos are free to see as many times as someone wants. It can be downloaded through other indirect means then why not provide it explicitly?

Is the issue here is legal or more related to revenues?

I think legal issue can be solved by providing an option to the uploader to make the video downloadable if he wants to.