If you were disturbing every single shipment, I would assume someone was tampering. Once I find a shipping route where none of the items were disturbed, I would know it is secure.
Right, if you are worried about covert tampering and every shipment shows up tampered, you have proven that someone along the shipping route is tampering. You react by distrusting any packages that were previously shipped without anti-tamper protection. And you find a different shipping route. This is costly, but it's a reaction to a threat you can prove to be targeting you.
The tamperer is probably better off not tampering with packages, unless they can do so in a plausibly deniable way.
