If the gunner ran out of bounds on his own, then I guess you'd be right, but if the return team is purposely forcing the gunner into or behind the wall, then it becomes a chickenshit move. The Jets aren't the only team that does it, the Patriots are well known for the practice as well. The fact that the Jets are rolling this jagoff under the bus is a sign that this practice is not widely accepted by the league.
Every defender uses the sideline. One of the first things a CB is taught is to use the sideline as a second defender. BUT, once you have blocked or forced your guy out of the field of play, you must allow him to return (you cannot maintain a block out of bounds or impede progress back onto the field of play) or you will face the same 15 yard unsportsmanlike conduct penalty. With the 1265 people that roam the sidelines in the NFL, it is not like they ever have a "clear" path anyway. Also, watch closely...the defender will get "blocked" out of bounds rather easiley then get back to the playing field in the most descriminate non-direct manner possible (sprinting
just outside the boundry for as long as they can without getting a flag).
Both sides push the envelope of legality when it comes to using the boundry.