Amazon subcontracts their delivery services, so there are probably additional factors driving Teslas decision.
My guess is the insane liability a car accident can create. Trucking companies have had to pay out millions, even for accidents where they weren’t at fault.
I would bet that if self driving software was involved in any accident, it is the company who developed the software who will be paying, even if someone else owns the car.
A big part of Amazon's "Delivery Service Partner" program is that it creates many smaller work-forces rather than one big one, which prevents unionization. On the other hand, Amazon can probably afford accident insurance.
The fact that robot taxi's won't form a union makes this rather unlike Amazon DSP's. I suspect the article is correct. There simply aren't net profits here for operators.