Like most DRM vendors — Apple and Google, for example — Amazon spends a lot of time implying and flat-out stating that it only uses DRM because the big dumb media companies require it of them. The reality is that DRM's primary beneficiary is the DRM vendor. Once your book is sold with Amazon's DRM on it, only Amazon can give your readers permission to move them out of the Kindle jail and onto another device of your choosing. Of course Amazon wants to force copyright holders and creators to use its DRM — it's a one-stop way of converting the writer's customer into Amazon's customer. Forever.
Remember: Any time someone puts a lock on something of yours and won't give you the key, that lock is not there for your benefit.
Please note that this is an Amazon contract and that Amazon is the one who is insisting on the DRM. That makes this an interesting contrast, IMO, with Bezos' statements that "If the rights owner wants DRM, we do DRM. If the rights owner doesn't want DRM, we don't do DRM."
The contract clause mentioned above is by no means agnostic on the topic of DRM. It unequivocally tells us that Amazon is the one who gets to decide whether the ebooks have DRM. It also tells us that Amazon _will_ be adding DRM to Kindle ebooks unless the other party can talk them out of it.
Amazon's position (on DRM) in this contract is far from agnostic, and it is in fact much closer to their stated position for audiobook DRM.
A Leaked Contract Reveals that Amazon Insists on DRM [Nate Hoffelder/Amazon]