If you think about it NYC and LA are the most competitive markets with privately owned/indie theaters and a multitude of other chains: Alamo, Pacific, Regal, Cinemark, arc light, etc all competing with each other. AMC now has an advantage by offering this deal and possibly enticing people that usually frequented the other theater chains. Now those customers will most likely stick with AMC since the sub only is for that. That increase in foot traffic will most certainly lead to sales increasing in concessions.
Also, I have seen various AMC employees posting on different forums saying that usually the premium showings of the movies were empty besides for MCU opening weekend. Now those will be filled and leave the regular 2D showtimes that people not on A*list are willing to pay for.