I believe youre misunderstanding how KMS works and is exposed. You probably want to look at the concept of “kms grants.” Thoese regulate which principals, including service principals, can use CMK materials. The customer controls those grants. There are also substantial public docs, and more available on request, around the implementation, certification, and compliance of KMS infrastructure. If KMS is insufficient for your needs CloudHSM is availble for something even closer to “hosted HSM” than “key service.”

In short IAM controls everything, there is no “back door” or universal admin access, and KMS is used to perform sensitive operations NOT handing secrets to arbitrary (internal or external) consumers.