To a large degree the system hasn't changed at Atlanta. The USA simply doesn't do transit. You have to enter the USA and then leave again, even if both flights are international. This means you have to have a US Visa or ESTA.
What has changed is the precise way this system operates at Atlanta. The international arrivals used to be in a terminal that was the last stop on the train from the landside terminal. To get to landside you had to go through the parts of the terminal that were airside and all the people you met had passed through TSA security. Their view was/is that the security screening you passed to board you flight into the USA is irrelevant. What happened was that you cleared immigration, then customs and found yourself in a quite small area where you were told to put your checked bags on a conveyor and put yourself through security. If you hadn't experienced it before you did just that. Trouble was that you then had to go through TSA and their liquids rule so any duty free was removed and destroyed. The system did mean that you were able to then board an onward domestic flight. Everyone else stayed on the train to landside and then had to find the other end of the conveyor and pick up your bags. If Atlanta was your destination fine. But, to continue on an international flight you checked in again and then went through TSA yet again!
The new terminal works on the same basis. Clear immigration, clear customs but then you exit to landside. That may seem to be different and for terminating passengers it is but the effect for international connecting passengers is the same because having rechecked your bag for the connecting service you have to pass TSA again and, you've guessed it, the liquids rule again.
If you "transit" the USA it is not advisable to buy duty free until you are about to depart the USA where there are duty free shops or wait until you're on the international flight.