Because people are stupid.
HIV is pretty much non-contagious as long as you don't exchange some sort of body fluid. Now, I don't know how you go about in your everyday life, but I don't routinely have people spill blood, semen or other stuff coming out of their body into mine.
But people are stupid.
Remember the H1N1 craze? Swine flu? Or any other of the sky-is-falling pandemics? SARS anyone? Yes, they are contagious. How many cases did we have around the US and Europe? Was it more than a dozen combined? People went apeshit over that crap. Mostly because they didn't have the first clue about it other than "oh it's killing people, watch out!"
And now imagine these people should interact with people who actually carry a deadly disease. No matter that there is no sensible way they could get infected, they WILL go bananas over it.
HIV is already a disease that puts a terrible weight on your psyche. Making these people outcasts for no reason whatsoever doesn't really help it.