FEET ON THE LINE AT A THROW-IN

Question:
As a referee, I have always been told that the lines on a field are part of the area of which they “contain”. However, this seems to be in conflict with the law regarding throw-ins and the placement of the feet of the individual taking the throw-in along the touchline.

I recently had a game in which I had to explain the lines are part of the area of which they contain and he brought up the fact that on a throw-in as long as both feet are touching the touchline in some form that the throw-in is considered legal. However he then pointed out that by my description, would not that be illegal since in a throw-in the player must take the throw-in from outside of the field of play, however the line is considered in play?

The only reasoning I can come up with for this is that at its most basic form the throw-in is a method of restarting the match and thus follows a slightly different set of circumstances or rules than normal course of play.

But is there any further reasoning as to why a player is allowed to be completely in the field of play when taking a throw-in (in the case where they keep both heels on the inside edge of the touchline) and yet the throw-in is technically taken to put the ball back in to play?

USSF answer (November 24, 2011):
The answer to your question lies in applying Laws 1 and 15 as they are written, not in finding reasons to doubt them. “He,” whoever “he” may be, was totally wrong in suggesting that having one’s feet on the line had anything to do with a dichotomy in the Laws. Your original understanding is correct. Your interlocutor is talking apples and applesauce, two different things, and creating his own muddled version of the Laws.

Law 1:
Field Markings
The field of play must be rectangular and marked with lines. These lines belong to the areas of which they are boundaries.

Law 15:
Procedure
At the moment of delivering the ball, the thrower:
* faces the field of play
* has part of each foot either on the touch line or on the ground outside the touch line

This is the Law and it is also tradition. Where the Law is clear, follow the Law; where it is not, do the best you can (including applying logic).