Recreational use of two particular drugs, which are major players in HIV treatment, is causing the development of more antiviral-resistant strains of the virus. A number of people who have never been on treatment are becoming infected with such resistant strains because of their drug use, or because of having sex with people who use ART recreationally. These people are far more difficult to treat than those who have non-resistant strains, particularly since individuals in South Africa may not have access to a wide range of other ART options.
In effect, people who are using HIV drugs to get high are paying for their good time with the risk of reducing their doctors' ability to fight any HIV infection they may acquire. That's bad enough, but as these resistant strains of HIV spread throughout the population, it isn't only the recreational drug users themselves who are at risk. It is also everyone they become intimate with, and the people those people become intimate with... and there is the possibility that novel strains of resistant virus could spread exponentially throughout the population. That's a pretty high price to pay for a few hours of fun.