The truth about YouTube Mailer

I am still wondering what the YouTube mailer is all about. I have received some comments on the earlier post about "the YouTube mailer being a scam". I didn't mean that the software is a scam, I was just curious on how it works and how it can not be a scam. A few people have told me that it works great. What I am wondering is, how do they double optin from the YouTube users? Is the list of users open to the public? Have the [tag]YouTube[/tag] users been asked to join this "safelist"? And if so, are YouTube / Google involved in this? I just can't seem to figure out how this software really works.