It is not illegal if it does not exceed the prescribed scope. It is illegal if there is fraud and other means.
Supervision fees are legal and reasonable fees that can be used to maintain and promote the normal application and use of patents. If the patent agency complies with relevant laws, regulations and policies when collecting supervision fees, and the laws and regulations followed include the "Patent Law", "Patent Agency Regulations", etc., and does not exceed the prescribed scope, then such fees The behavior is legal.
If a patent agency uses unfair means such as fraud, misleading, or coercion when collecting regulatory fees, or violates relevant laws, regulations and policies, then such charging behavior is illegal.