When I spoke to Amazon about this, they did of course block and defer. It is, apparently, consistent with the policy on every product they sell that has a return policy, no questions asked.

I brought up the sinister case that readers buy book 1 of a series, return it, buy book 2... and work their way through an entire series for free. This breaks the returns model quite solidly.

Amazon said that if you suspect one account is doing this, contact them and they will investigate. First off, it's very hard to prove from anonymous sales reports. Am I sure that out of the 10 books I sold today, it was the same account returning the book? No I'm not sure, and can never be.

I know million dollar a year authors talking to Amazon about this. I doubt Amazon will change. They might cut a deal for their whale authors.

Finally, abandon KU and go as wide as you can. I sell more books on Google than I do Amazon. At some point I may be able to quit Amazon entirely.