I have been with a company that received a nonconformity for this very reason. Registrars provide you with a service. You pay for their audits and they provide you with assessments, audit reports, and certifications.
Even the FDA or other regulatory bodies provide a service. You pay them and they provide you with registration. Sure, this is not optional and these regulatory agencies are often the sole-source of these registrations, but they still provide the service.
For regulatory, you can justify a lightweight supplier approval by stating they are the sole-source and it is legally required. You certainly aren't going to be able to exercise controls over the FDA, and no reasonable auditors would expect you to.
For registrars, for which you can choose one of many, the situation is a little different. I typically approved registrars based upon their certifications, years in business, etc.
How these fit into your system will depend on how you have your supplier classifications set up. I believe you could list these as high risk if you wanted, and justify the lack of typical high-risk controls based upon the uniqueness of these types of service suppliers.