Definition
From ICAO (in accord with the ADREP 2000 standard as defined by an international working group chaired by ICAO and implemented in version 4.2.6 of ECCAIRS)
Radar Control - "Term used to indicate that radar derived information is employed directly in the provision of an air traffic control service".
Similarly, CAP 493 defines Radar Control as "Term used to indicate that radar-derived information is employed directly in the provision of air traffic control service.
Furthermore that the airspace within which the aircraft is flying determines the type of surveillance service available:
Controlled Airspace = Radar Control Service
(Outside Controlled Airspace = Deconfliction Service or Traffic Service)
So, in my mind, the term is defined. Whether "Radar Control Service" is taken by some to mean something more than the above, IMHO, is down to level of initial/refresher training and/or examinations.
I hear your argument relating to the implied plain-English meaning of "control service" but would argue that if you are in controlled airspace you must comply with instructions (or advise that you cannot) then your flight is being "controlled".
Also, and perhaps a little pedantic (put that down as an ATC trait!), most dictionary definitions of "control" generally go something like:
Noun
The power to influence or direct people's behavior or the course of events.
Verb
Determine the behavior or supervise the running of.
Seems to me like a reasonable phrase, even in plain-English?
Regards