Magic Johnson has been an advocate for the HIV community for a long time. He has worked hard to spread awareness of the disease and to raise money for patients and their families. Other celebrities with a positive states are always working together in beneficial ways to find this terrible disease.
No, being HIV positive does not negate the message in favor of HIV prevention campaigns. Being HIV positive can be used as part of their message in favor of HIV prevention. They can use their experience with HIV to demonstrate why HIV prevention is important. Personal experiences can be a valuable tool for campaigning for HIV prevention.
People like Magic Johnson who are alive and well years after their HIV diagnosis are actually helpful in teaching people about preventing HIV in themselves. They are inspirational and can talk about how hard the journey is even though the death rate has thankfully gone way down over the last couple of decades.
I do not think that high profile celebrities such as Magic Johnson dilute or diminish the potency of HIV preventative campaigns because they have received an HIV positive diagnosis. I think if anything they bring more attention to the cause which does more good than harm. They are a good voice of prevention because they somehow failed to prevent HIV and are paying the ultimate price,
No, celebrities do not do anything of the sort when they share they have received a positive HIV diagnosis. The person who asks this must be very young, because I remember when Magic Johnson announced his HIV status. A lot of people who thought HIV and AIDS was just a gay disease got educated that day.