Not naive at all and a very valid question.
Of course the F/O must be allowed to learn from their mistakes, just as they would learn from the things that they perform well. Captains dont stop making mistakes from time to time either, simply by virtue of the change of seat, or the length of time they have been occupying it. However the Captain is the Captain because they have aquired the necessary experience and been tested to satisy the basic requirements of that position. There are additional responsibilities and authority that are encumbent with the promotion. Perhaps the most obvious one is that you are then "in charge" and the de facto manager of the flight. Regulation and statute places various legal obligations on you as a result of that postion, and the company employing you, expects a proper discharge of these obligations as well as additional responsibilities that it will define in its operating manual.
The other crewmembers operate under your authority in the dischage of their duties. When everything runs well (as it usually does,) then your role as a manager is a fairly easy one. When problems occur, it is often a case of allowing the crewmember to suggest and effect a solution usually through you. Sometimes the problem or the solution requires direction or input from you as the manager. On rare occaisions you may well need to intervene and initiate an entirely different solution. It is this latter case, and the borderline margin leading up to it, that is really the subject under discussion here.
Certainly nobody should ever be discouraged from performing a go-around in an unstable approach scenario or where the outcome of a landing is in any reasonable doubt. If you are unhappy, then throw it away and start again. However in Public Transport operations there are other serious considerations as well. It is very important (and just as much part of the learning process) that developing mistakes are pointed out and corrected in a timely manner. The F/O will be aware that it is not just a case of satisfying the Commander, but also his responsiblity to the hundreds of paying guests down the back. Whilst a go around should be a normal if seldom practised manouever to the pilots, it will be less happily received by the passengers who might have to wait some considerable time for an explanation. As such, the situation that might lead up to a go around, should not be allowed to develop simply to satisy the F/O's need to learn from his mistakes. That might be the end result, but subjecting the passengers to any discomfort is most certainly not to be entertained lightly, and definetaly not something any responsible captain would normally allow to occur.