There are certain professions that would require the disclosure of HIV status. These are linked to the jobs where blood and bodily fluid are not uncommon. However, common workplaces do not require such contact, and even if an incident or accident were to occur, the likelihood of infection is reduced just by basic common sense. There is no need to advertise the HIV status and cause fear and alienation.
Confidentiality in the work place should definitely extend to the results of an HIV test. There is absolutely no reason why anyone at work should know your HIV status, even your boss. There is ample evidence that someone with HIV is of no risk to anyone who they come into contact with on a casual basis.
If someone works for a company and has HIV, there is no reason to tell everyone about it. Still, to this day, this virus is completely misunderstood and it scares people more than it should. This is an STD that is extremely hard to get, never mind through interactions in the workplace.
Yes, confidentiality in the work place has not extended to positive results on an HIV test, because HIV is not something that other people can contract through normal circumstances. The person is so likely to be discriminated against because of the status, that society is better served by not telling the coworkers.