It's not only clumsy but also difficult to comprehend. But the organization itself allows the use of such a term because the membership is based on customsterritory instead of sovereignty.
Without Customs approval, no persons passing through the territory may leave in the territory the articles they bring in.
过境人员未经海关批准，不得将其所带物品留在境内。
3
If goods originating within Chinese territory are purchased from outside Chinese territory for import, Customs shall levy import duty in accordance with the Customs import and Export Tariff.