I think the reasoning goes something like this: The anus should be assumed to be an unclean portion of the body, with the exception of directly after washing i

I think the reasoning goes something like this: The anus should be assumed to be an unclean portion of the body, with the exception of directly after washing it with soap and water, and should always be treated under that assumption. By using paper in between the hand and anus when wiping you are attempting to avoid cross contamination with the hands, which is a part of the body that is understood to be much cleaner. Simply avoiding contact between the two areas is the most direct route to maintain this distinction. Washing your hands with soap and water after wiping serves as a further action to maintain the usefulness of the hands. You will notice that I highlighted soap above, I do this because I believe it is the key factor often missed in discussions like this. You can argue that hand + water can make your anus appear cleaner than hand + paper, but without soap it is at best marginally more sanitary for your anus, and significantly less sanitary for your hand. There is a reason we clean our bodies with soap and water, not just water.