I was at a Subway post security in some airport that I can't remember right now, and the knife that they used to cut the sandwiches was chained to the counter. I would imagine that most of the prep work that would require knives in the back would be done offsite. And every time I have seen a shop worker unloading merchandise it was from a "tote" - so no sharp objects necessary. I would be shocked if anyone were allowed to have a box knife back there, but I would be equally shocked if there weren't a whole bunch of "UA" (unauthorized) ones floating around post-security.