Tipping is a nuisance, something that goes in the "Con" category in a Pro & Con analysis. The vendor is implicitly warning you that you shouldn't count on getting good service included just because you officially paid for it. They're warning you that they have no real control over their service providers, and "it would be a shame if something unforeseeable, like getting caught in traffic, were to happen to a delivery you were counting on", so you'd better work something out with them privately and hope the "service" provider is pleased with the result.
Like many people, I have a lot of options when it comes to who I buy from. I now have to pay sales tax to Amazon, and there are often other vendors who are cheaper, or charge no sales tax, or have stores five minutes away where "shipping" is instantaneous and always free for even the smallest purchase. Even so, I'll often pay more and buy from Amazon anyway because of their service. Anything that reduced their service quality would shift some of those purchases away from Amazon.
I haven't seen this new Amazon system for myself yet, so maybe there's no real problem, but if "free" 2-hour delivery turns out to require air-quotes around the "free" along with a nudge, nudge, wink, wink, "it would be a mistake to disappoint me" attitude from the delivery guy, I'll use the system a lot less.