I didn't know a one line answer and a link was a Book???
The factors they use are part of the statistical model to balance things out....people who left the work force because of retirement are people who clearly have a long work history and will no longer be seeking employment thus will probably not be eligible to employers but they are more than likely capable of working or continuing to work, and some do at a reduced scale. Thus being capable of working means they are leaving the work force....
Those people capable or uncapable of work do to various reasons who Do not seek or want to seek work are not going to be seen as possible employee's. If you are by choice not seeking employment or have no drive to do so then you are clearly not unemployed you are a arse sitting lazy butz. Hence they should not count. If you have a legit reason for not working then also you should not count either.
Now does it make a bit more sense. I'm not saying the Model is perfect, but it is what they have used for awhile. And there is some reason behind it.