I wonder what people would think of an occasional "postal zone dump" employing the same kind of dragnet but for communications on paper.
You don't have to wonder about this, because this is how it is now. The headers of all snailmail (the wrapper of the packet) are machine-logged. Those of us who are technically savvy always suspected this, since we found out that scanning is used for routing. Some of us, like myself, even mentioned the possibility to our postmasters and were told that they were simply throwing this data away after collecting it. But anyone who knows anything about anything knew that this was massively unlikely.
So, given that this is already happening for literally every piece of mail being sent, just like it happens to literally every piece of email which traverses a long-haul link, why do you wonder? That's how it is right now.