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.
We're not told that one way or another, honestly. I'm not sure I'd assume that they're not just doing it on their own considering the bad press they're getting in regards to treatment of warehouse employees.
Do YOU want your company's clients investigating you directly instead of talking through your company? This is not how B2B works and there's a reason for it. A good company is going to protect it's own employees from abusive practices and if Amazon isn't working with the company, it's abusive. If it were my company and I found out Amazon was doing this, I'd tell Amazon to piss off and see what my lawyers said about what I can do in retaliation.
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.