Based on the decline of Amazon UPS deliveries over the past couple years, UPS is becoming less and less important to them. For a while, Ontrac was doing most of my Amazon deliveries, but more recently, Amazon's own delivery service has been delivering most of my orders (not sure if they are really Amazon drivers or if they are contractors). UPS is still doing some deliveries (seems to be only products that aren't available in nearby warehouses), but not nearly as many as they used to. When Amazon first started using Ontrac, Ontrac's delivery service was so horrible that I think Amazon did it only so they could squeeze more money from UPS and Fedex, though Ontrac has improved a lot since then.

Recently Amazon has starting using the USPS to deliver more packages (or at least the last-mile delivery), and that service has been disappointing, I can usually count on a USPS package being delivered at least a day late, and more than once, it hasn't shown up at all, despite being marked as "delivered at front door".

In any case, while I would think that any unhappiness between UPS and Amazon would be resolved through their contracts, I doubt that UPS wants to rock the boat with Amazon, because even a small piece of the Amazon delivery pie is a lot of deliveries. So in this case, where workers health and safety is in question, it's good that the government stepped in to prevent UPS's desire to keep a customer happy from getting in the way of their worker's welfare.