why am I charged for KMS?

Hello.

KMS keys managed by AWS are charged on a pay-as-you-go basis (based on the number of times the API is executed), so if you are being charged a fixed amount of $1 per month, it is likely that some kind of customer-managed key has been created.
https://docs.aws.amazon.com/kms/latest/developerguide/concepts.html

What charges appear on my AWS bill?
Since KMS is a region-level resource, be sure to check the KMS consoles for all regions in the Management Console.
https://aws.amazon.com/kms/pricing/?nc1=h_ls

Each AWS KMS key that you create in AWS KMS costs $1/month (prorated hourly). The $1/month charge is the same for symmetric keys, asymmetric keys, HMAC keys, multi-Region keys (each primary and each replica multi-Region key), keys with imported key material, and KMS keys with a key origin of either AWS CloudHSM or an external key store (XKS).

For KMS keys that you rotate automatically or on demand, the first and second rotation of the key adds $1/month (prorated hourly) in cost. This price increase is capped at the second rotation, and any subsequent rotations will not be billed.

You can check which region your KMS charges are incurred in from your invoice as shown below.