A lot of rules but then a lot of grey areas - and that makes people take undue advantages.
However, I am not sure about the effect of the cost of having this system - additional resources, queue time, and all. But yes, there has to be a check. If I remember well, in Hawaii, we had to pick up the bough duty-free items at the airport. The duty-free payment was made at the store, but had to collect it from the airport. The additional cost in such cases goes to the seller.