Allowing unrestricted, public access to cloud services could open an application up to external attack. Disallowing this access is typically considered best practice.
In AWS Console -
In Terraform -
References:
https://docs.aws.amazon.com/sns/latest/dg/sns-access-policy-use-cases.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/sns_topic