Interesting point which brings up a sad double standard.
Though I'm negative, I would not expect anyone to believe me. Anyone who is smart, shouldn't. It's also unrealistic for me to assume that someone should believe me because I say so. If I get into a long term intimate relationship, I would fully expect to go and get tested together. If we both turn out to be negative at the same time, that would be good information to have. Depending on the trust of the relationship, it may progress depending on the dynamic of the relationship.
Now, if someone posts that they are positive, there is both a good and bad aspect to this. Granted, from my view, the 'bad' aspect is minimal. I wouldn't hesitate to date and get into a relationship with someone who was positive as I had in the past and simply addressed it with reasonable and rational caution.
If you honestly state that you are [HIV] positive, then there are people who will not want to date you. As harsh as that may seem (the 'bad' aspect), it will save you a lot of pain later when you disclose it and it ends a relationship you've invested in.
Basically, it gets down to the moronically simple: "I don't want to date anyone that doesn't want to date me." How often, though, I've seen people completely ignore this rule as if Hollywood magic is going to turn the frog into a prince before our eyes. You'd be better off buy lottery tickets as a retirement investment.