While it does seem odd, but if the person was doing/planning/carrying something illicit that they wanted to keep them away from the general populace before they had a chance to clear them. I.e. the plane may have had only a 100 people on it while the immigration/customs area had a 1000 people. Further, not all people may be been connecting so perhaps for those staying they wanted a quick check. And because it was unknown who that would be everyone gets checked.