Reviews are also removed when they are reported as being written, provably, from the same IP address (if it looks like someone is logging in with fake accounts and writing fake positive or fake negative reviews) or from family members or others who may stand to benefit from the book (either by eliminating it from competition by writing negative reviews, or by boosting sales by writing shill reviews). It’s actually common for books ( I am not referring specifically to this one) to receive fake positive ratings (often purchased on Fivrr) as a part of a marketing plan so there is a chance that Amazon removed reviews believing (correctly or not) that they were incentivized reviews in some way. It is sometimes possible to contest review removals and get the reviews re-instated. We’ll see. I am not aware of review removal based soley on controversial content - not saying it doesn’t happen but am saying that review removals I’ve seen have always been due to claims of financial interest, multiple sock accounts, family members, or author rivals trashing each others books with fake negatives.
Seems rather common with book reviews, but in general Amazon runs a very tight ship.
Wish all e marketers were as much of a pleasure to use.