Opting Out vs. Opting In

"Opting out is the what happens when a content producer discovers that their content is being offered up (via RSS) on another site. They can write to the site owners and ask that their content be taken down. This is, of course, a time consuming activity.

Opting in, on the other hand, requires that the site ask content producers ahead of time if they would like to be included in the directory. This is also a time consuming task only the time is spent by the directory operators and not the content producers. It inhibits the directory from growing rapidly. Directories, in order to be effective, must be all inclusive." (http://loadedpun.com/2006/12/17/who-owns-a-feed.html)