Because most people assume it's people flying to/from that airport, not just getting off for security/customs. It is the way it is, I'm not trying to suggest anything underhanded. As I pointed out it happens at plenty of other places. Other examples are JAI in BRU, KAC in LHR, SIA in FRA, FPO in DUB. It just needs to be kept in mind when analysing annual figures for growth in tourism or business travel.
I wasn't trying to smear EINN in any way, doing that normally starts a catfight on here.