Believe it or not, ref6983 and jg-me's thoughts also occurred to me while I was out mowing the yard this morning (after my comment at 9:16). Yeah, determining which players are restricted and determining such player's adjacent sideline's might be, and apparently are, two different things. But, I'm not sure it is a good thing. Same with the difference between the landmarks for linemen and backs. Yeah, the landmark for the 'tackle box' is the snapper, whereas it is the middle lineman for linemen. Granted, for the strong majority of situations, they are the same, and the rules work just fine. But, as TxMike points out, swinging gate formations, etc., can put the middle lineman far removed from the snapper. If the middle lineman was used as the common landmark for all of these things, then all of these issues disappear.
But, at least with RR's interpretation, we know which is the adjacent sideline, and that's a good thing.