The legal justification is that it is the same as police setting up a road block and reading license plate numbers as traffic flows by, or in the case of packet

The legal justification is that it is the same as police setting up a road block and reading license plate numbers as traffic flows by, or in the case of packet inspection actually doing "drunk driving" stops, which are legal. Edit: I'm actually wrong here, I looked up the source I originally read this on and realized I misinterpreted it. It's only one possible answer, not the answer: there is no case law, it hasn't been tested yet.