People Management: Feedback Mechanisms

I've always wondered why line managers prefer to appreciate or rebuke their reportees in private. In organizations and countries where I have worked, I found it peculiar that some line managers preferred to say nice things in public, but harsher ones in private, perhaps in a meeting room. I dislike this practice of discriminating between positive and negative feedback and since most people in modern organizations work in teams, see no reason to withold information about a person's performance from the rest of the team. When I was line manager, having 3 different teams reporting in to me, I always preferred to be open and frank and discuss everything out in public. I found that many of my reportees preferred this as it meant not having to 'suffer' through the agonizing process of booking a room, going in at a pre-designated time and spending more than a required length of time on something that could have mentioned in passing.

The responses from my own line managers however, were usually unsupportive. What I had begun doing seemed to them to be incorrect and improper. While they understood over time where I was headed or why I prefered to give 360 manager feedback in a more open way, I do not think they supported me in this endeavour through my time working with them.

I do not think I know of any organizations, or managers, that prefer to act in this way, and if there is a specific reason why this is so unpopular?