Amazon may do it because others are doing it and as you say they don't have to because they are already trusted and at the top of the heap. And further they d

Amazon may do it because others are doing it and as you say they don't have to because they are already trusted and at the top of the heap. And further they don't want to empower other sites as being legit by featuring a design element (the green at the top) that others can easily copy. An example of this? Let's say back in the day you were the market leader and had "intel inside" (the processor). But you didn't use the 'intel inside' logo because then others could have the same sticker and the public would think 'oh ok it's the same stuff in brand X'. Manufacturers frequently come up with different names for product features for a similar reason.