I can choose what books to place on my shelves; so can Amazon. I might not agree with their decision.
Once a contract is signed and sealed, Amazon has no right whatsoever to renege on it without both consent from the contracting party and a full refund.
Now you are talking contract law, not censorship. I'm wondering if the contract might not cover this.
Or if the contract allows Amazon to renegotiate terms of the contract by itself, for any reason, with no exceptions allowed by the consumer allowed without binding arbitration, at the consumer's expense.