I dunno that there has been any official DL policy on this, but I think it is a carryover from many PMNW crews. I def appreciate it when I am in F, and don't really mind it when I am in Y. Most of the FAs that I have seen wisely allow Y pax to start deplaning after most of F has gotten off rather than holding them for the one or two F pax that inevitably straggle behind.