Amazon cracking down on newsletter recommendation services?

A lot of self-published authors have recently noticed a problem with some of the promotions they've been using to boost sales and ranking of Kindle ebooks on Amazon. Once-safe promotional newsletters like Bookbub have apparently resulted in "rank stripping" and accusations from Amazon that the authors responsible have been using forbidden methods to boost their rank - a hallmark of the "botters" who use illicit methods to take over Amazon categories and rank in cash.

This is a big deal for legitimate authors, because they depend on these services to increase their visibility and sales. It's also a big deal for Bookbub and competing services, because they not lose sales from potential authors (a placement in the newsletter can cost hundreds of dollars) but also because they lose out on Amazon Associates affiliate income. Some of the newsletters have hundreds of thousands or millions of subscribers, who like the links to download free or discounted Kindle books.

Why is this happening? Lots of people seem to think Amazon is incompetent, or its automated systems for flagging botted books is set too high, and is throwing out the baby with the bath water:

Amazon desperately needs to be some people on this and stop relying so heavily on bots. The whole situation illustrates the dilemma of trying to beat scammers mostly by automated means. It simply can't be done. Either a lot of scammers keep on scammer with impunity, or innocent authors get caught in the anti-scam measures--or, as seems to be the case now, both things happen at the same time. 

I am going to offer an alternative hypothesis about why this is happening:

Amazon's prohibitions against "rank manipulation" have been recently expanded to include unauthorized promotional tools, regardless of whether authors consider them to be legitimate.

I don't have direct evidence of this, but I believe the following activities by Bookbub et al likely run counter to Amazon's own priorities:

1. Maintaining a giant list of Amazon customers
2. Encouraging behavior that runs counter to Amazon's own navigational and promotional tools
3. Diverting money away from potential Amazon Marketing Services (AMS) spending
4. Running up affiliate payouts
5. Providing potential cover for actual botters and scammers, who mix in among legit authors and hope they won't get noticed.

We all know that Amazon values iron control over its platform, and wants to extend its power. Newsletters, which are basically mini platforms that depend on Amazon to survive, may be causing too many problems, and not generating enough value in return.

Does Amazon really care if Bookbub et al loses business, especially if skittish authors decide to stick with AMS? I suppose you could argue that these newsletters increase sales, but from Amazon's POV those sales might have happened anyway -- and in a more profitable way for Amazon -- if customers just used Amazon search, Amazon recommendations, Amazon sponsored ads, and Amazon newsletters to find good deals.

That's my hypothesis, anyway. And I have to admit there are a few things that go against it:

1. If Amazon really wanted to crack down on newsletters, it could do so in ways that don't rile up the author community and increase resentment. A change in stated policy ("you can't use affiliate links or newsletters to increase sales") or cease & desist letters to newsletter operators based on real or supposed TOS violations could effectively end these types of promotions in a very short period of time.
2. Customers really like the newsletters. To Amazon, customers are king, so pissing them off is not a good idea.

Nevertheless, if I were Bookbub (or any one of the other services that have a large Amazon customer list) I would be concerned. A business model that depends on a larger platform to succeed is incredibly vulnerable to being shut down or seriously harmed if the platform decides to change course. We've seen it happen before when Facebook cracked down on social games in 2010-2011, as well as Twitter a few years later when it decided third-party services built on the platform had to be reined in. Amazon can and will do the same if it feels newsletter promos are causing problems.