Why would a restraining line plane need to extend beyond the sidelines?
K/O, A-35. A1 kicks the ball from a tee into the air. The ball has not touched the ground, and has crossed the sideline at the A-43, and is over out-of-bounds territory at the A-44 when A2, with his feet inbounds, reaches over the sideline and bats the ball back into the field of play, landing at the A-42, where A3 recovers the ball. No player of Team B was in position to catch the ball.
Ruling: Illegal touching by A2 (and A3). B, 1/10, A-42.
Without this language, it would be possible to interpret the rule to mean that illegal touching only applied over territory in the field of play.