Essentially, Amazon wants to shift their current royalties calculation methods. Right now, authors get paid full royalties for every book purchased in which a reader has read up to or past 10%. This already suggests a fairly high level of surveillance for readers that ostensibly own the book they are reading (DRM notwithstanding). The change will involve a pay-per-page model, which means they would need to be very tightly scrutinizing reader behavior. So far this is really only affecting self-published authors on Kindle Unlimited (which I am biased in thinking are pretty into the whole commercial aspect of the business, considering how terrible a lot of those books are).
Amazon isn't the first to do the 10% thing- Oyster and Scribd I believe do the same. But they are subscription services- you don't own their books, you are borrowing them, and with so many options available, they have to figure out what is being read before they pay for something no one is using.
Arguments about commercialism in publishing will continue to rage, as they always have, but I do find this sort of payment option baffling and counterproductive, especially as it comes from a preset fund.
EDIT: As some commenters have pointed out (WITHOUT calling me out at all, you guys are awesome), this is ALL about subscription. I think skimming articles at work makes me miss the details in an important way. But I still find the thought process fascinating, and the implications for future services, or changes to other models)