Do you play nice with Amazon's Terms of Service?

Yes. According to Amazon’s Community Guidelines,

authors and publishers may continue to provide free or discounted copies of their books to readers, as long as the author or publisher does not require a review in exchange or attempt to influence the review.

On BookSirens, readers are NOT compensated, incentivized, nor required to leave a review. They are also NOT influenced to leave a biased review. Readers simply use BookSirens to discover complimentary review copies and leave honest, voluntary reviews. 

Furthermore, according to Amazon's Promotional Guidelines, a review is not permitted if:

The customer is a member of a review club where sellers or brands give the customer ratings or refer the customer to other sellers or brands based on their review

Authors or publishers (i.e. "sellers or brands" in Amazon's definition) cannot rate nor refer reviewers (i.e. "customer" in Amazon's definition) to other authors or publishers within BookSirens (i.e. "review club" in Amazon's definition).

The customer is a member of a review club where they are asked to register their Amazon public profile so that sellers or brands may monitor their reviews of products offered through the club

While reviewers (i.e. "customer" in Amazon's definition) are encouraged to share their Amazon public profile with BookSirens (i.e. "review club" in Amazon's definition) for vetting purposes, BookSirens does not disclose this information to authors or publishers (i.e. "sellers or brands" in Amazon's definition) for monitoring or any other purpose.

If you are considering the Promote Plan, read this.

If your books are in KDP Select / Kindle Unlimited, read this.