This has been standard at airports that have int'l-to-int'l connecting passengers since well before 9/11. Even prior to 9/11, it was a big blow to the prestige of an airport and/or country that was the departing city of a hijacked or blown up airplane. Since inbound int'l passengers were screened by an airport that was under the control of a different government and set of policies, there's a notion that no country wants to take the chance that inbound passengers have been properly screened. This is especially true in regions where the neighboring countries are not always on the best of terms - it's not beyond the realm of possibility that an inbound passenger arrives with something concealed on them with tacit awareness of the departing government.