Unstated major premise: Buying electric vans is a step in the right direction in the first place.
I'm thinking here of the lessons we've been learning recently about recycling: It would appear that we now need to come to terms with the possibility that recycling has done far more harm than good, by making it easier to justify excessive packaging waste while at the same time mostly only giving consumers the impression that what they put in the recycle bin wasn't going to waste.
I could see something similar happening with electric delivery vans: Perhaps, by ostensibly greening the most consumer-visible part of the process, they can make them feel better about buying into a way of getting their consumer products that relies on a much less efficient supply chain than the one used by the system it's supplanting.
Perhaps, if we dig into it, we'll discover that Amazon is being pennywise here, because properly dealing with the pounds would require rolling back the "free 2 day shipping of pretty much anything to pretty much anywhere" guarantee that underlies so much of Amazon's current business model.