As the top comment suggests, if Facebook runs face recognition on pictures of you and those pictures contain other people, there is no guarantee that they have opted into the program as well. You wouldn't just be making a choice for yourself.
Well, either Facebook doesn't have training data for the opt-out users, or they do, and they use it to ensure that those users aren't publicly tagged (my paranoia says that they're still tagged internally, regardless of the option that's selected).
Either way, it would only need to identify those users who have actually opted in.
