This system actually predates the TSA. I think they argue that by entering the sterile area (i.e. walking through the metal detector), you give implied consent to any and all searches that they would like to do until you leave the sterile area. This is how they get away with the wasteful indignity of gate screening, and the costly nuisance of post-customs screening for arriving passengers.

What if you decline the search? I don't THINK there's anything they can really do, but I suppose that your unwillingness to be searched would create probable cause, thereby justifying a required search.

It's especially troublesome in Atlanta. And I'm sure they've thought about it. The TSA re-clearance takes a long time, the non-TSA assistant staff are awful, there is no elite/first class line, etc.

If I understand correctly, Keflavik has the same problem. This is an issue with airport design.