I've also seen this, and was puzzled.
Presumably there are both audit and stock issues and therefore explicit procedural guidance. Can they process the transaction themselves or must it be with the consent of and payment through a csm/csd? If so presumably you'd normally expect them to be taken and packed with the other person there?
Staff all have to have a receipt for any purchases and they are processed through the same machine that we use for selling tax free goods to the customers.
There are strict rules and guidelines and she will most likely have paid the crew member at the rear of the 737 before taking her purchases.
It is very unlikely that any cabin crew member or flight crew member, for that matter, would take off any drinks or tax free goods without a receipt because it would be grounds for instant dismissal.