Yes, but it's their (and their customers') property getting stolen. They're completely within their rights to find these people and take legal action, regardless of if they work for them or not.
They don't work for Amazon. That'd be like some other company coming in to test you at your workplace instead of going through the proper chain and your boss.
But supposing they find a thief. The only thing that they can do is file a police report and sue. All the driver has to do is say "I left it on the dock to get picked up" and that's it. Word against word. Why? Because the employee signed absolutely no agreement with Amazon.
And that's the important part. The employee signed no agreement with Amazon at all. So Amazon can go smoke a big fatty and get glad.
I'm not arguing that Amazon shouldn't look into it. I'm saying that they should follow the proper channels and work with the company directly and let the company deal with it.
The companies clearly haven't been dealing with it in the first place, either because they didn't care or they could never find the evidence to prove a particular driver stole a particular lost order.
What they do when they find the drivers is up to them, and taking action through the companies is probably their best bet if they don't think they have enough evidence to sue, but they weren't going to just sit around and keep asking these delivery companies to start investigating something they really couldn't give enough of a shit about. They took matters into their own hands, and that's not a bad thing.
They don't work for Amazon
Not completely true. Amazon has been working on growing its own delivery services for at least 5 years now. It's still small, but I see Amazon trucks fairly often on my hour long commute in SoCal. About 20% of my packages come through their service and they do cool things like take a picture of your package when delivered, but I think they stopped doing that recently
The drivers could be working for a subcontractor and that company may have language allowing it in their contract with Amazon. Amazon is investing a ton of money in their own delivery service so I'm not surprised they are doing this. They weren't satisfied with USPS, UPS, or FedEx because of damages to packages and lost shipments so they decided they wanted more control in the delivery aspect. They also have enough distribution centers that they can control more of the logistics side now but they don't have a fleet built up or aren't willing to build one so they're just using subcontractors. FedEx also uses subcontractors for small package delivery.
I see a lot of trucks around here with the Amazon logo on the side. They are also trying to get people to open Amazon only delivery services.
They could use this as a basis for continuing to have a business relationship with the third party. It is Amazon's responsibility to make sure they partner with reliable third parties so their customers get the product they paid for.
I can't disagree with your viewpoint, particularly.
Not their drivers so not their responsibility to "test" the drivers. Period.