Welcome to the TSA and air travel, where the only standards are NO standards. I totally agree that if you are the only person with the key to your firearm case, it is completely illogical to place the tag inside the locked case. If TSA conducts a secondary search and the tag is not visible to them, they have no idea whether the firearm was declared or not. I put most of the blame on the airlines for poor training of their ticket agents. I have found absolutely nothing in the TSA regs or US CFR addressing the declaration tag location. Even so, common sense should dictate where the tag logically should go..
Oh wait, nevermind.
As far as the differences in how TSA treats your bag at different airports, it depends on where the TSA scanner is located. Airports were never designed for these large heavy devices so they have to be placed where there is room and the floor can take the weight. Sometimes it's in a different area and no 2 airports are exactly the same