The article reports that the Thai government (the Thai Ministry of Health) has cooperated willingly with this study. Thailand was an early country in Asia to have a high AIDS infection rate, and has been a leader in that part of the world in working on strategies to find community approaches to preventing the spread of AIDS and to find medical preventives and cures for AIDS.
They probably sought them out for their high incidence of HIV -- it would obviously be unethical to specifically infect people to test the vaccine.
To add to this, at the time the trial started, Thailand was one of areas with the highest infection rate. It's common practice to do a trial where the disease is the worst.