At a lot of airports the standard of security at that particular airport means nothing at all as arriving and departing pax. can mix freely airside (LHR Terminals 1,2,3 and most US airports spring to mind). Take a look at some of the airlines that park up on the ramp at LHR and ask yourself if you trust the airport security at the departure point of that aircraft as much as you trust LHR's. You had better, it's the same thing as long as inbound and outbound flights are mixed.