In my company we use reseller billing as AWS do not have a local billing entity. The reseller owns the organization's root account and we do not have access to it. Every subaccount creation require a support email to the reseller.
free as in: AWS does not charge you
not free as in: you have to manage it (for example give a CI role access via OIDC, create a role for you to assume to do stuff via the console, etc)
My favorite factoid about AWS accounts is that there's a global, hard rate limit for their deletion. It's actually a pain point for us.