Who knows? Let's say they wanted to do some shady shit, is writing an email(with the possibility of discovery) on that, is the only way to go about it?
I really don't know why I'm being so generous to AMZN today
I would expect at least something that hints at it, e.g. the absence of another good reason, a hint at other reasons, "synergies with" mentioned etc.
Of course, as you mentioned, there could still be such motives, but with this trending on HN I expected some bombshell where they more or less obviously state that, not such a mundane assessment.