It could be they are just trying to speed the security process up? On many occasions I have been delayed at security while a long haul flight crew passing through in front of me spends ages transferring items from one bag to another to satisfy the idiotic hold/cabin baggage rules. Maybe they're just trying to encourage(?) crew to have the right sized bottles in the right place BEFORE going through security to save holding up everyone else in the queue while you repack your luggage? I'm not defending the stupid rules - it still pisses me off I have to buy milk and water etc at BAA airside prices - but it could be they are trying to improve the often long and painful security process for the rest of us?