Congress can vest the appointment power of inferior executive offices in someone other than the president (attorney general, judiciary).
As long as it does not interfere with the President's core Article II duties.
(Special Counsel did not impede this -- consider the nature of the office, but not necessarily exact reasoning of Humphrey's Executor).
*SCOTUS says Special Counsel is an INFERIOR officer because they can be hired by a higher official (AG), and are limited to the scope of their investigation. Thus, appointment by judiciary is allowed. Also could be subject to "for cause" restrictions on office (independent).
Olson, who was withholding documents, was subpoenaed by the Special Counsel, who could only be removed by the AG.
Argument that principal officer because there is no limit to who they can investigate, as long as it is tied to their investigation.