I know why they are doing this
First off, I don't know about various posters, but I'm not a consumer, I'm a customer (or not -- I don't subscribe to Amazon's book service.)
I know why they are doing this. Or at least strongly suspect. I can think of two good reasons.
I've seen, time and again, self-published "books" that are like 10-20 pages long. This is not to denigrate these books, but I really don't see it as fair for a short story to be paid out the same as a full-length novel.
I've also seen a few books in Amazon, where the preview made it clear it was gibberish. Not merely badly written, but apparently text from one of those spam-web-page pseudo-English text generators... the ones that make text that at casual glance appears to be English but when trying to read it it's nonsense and sentence fragments. These guys shouldn't be paid at all, but this way when a reader glances at 1 page, says "WTF!", and moves on, they at least will be paid for 1 page instead of a whole book.
Of course, the devil is in the details. It's probably possible for Amazon to set a per-page rate so typical writers actually make more than before, while cutting down payment on very short books and "spam page text" books enough that Amazon still saves money. But, it's of course also possible for the per-page rate to be set way too low so everyone loses but Amazon.