I think it would unethical if the participants in the study were asked not to take any other antivirals.

In that case, someone at high risk for HIV infection could be prevented from getting treatment because of the study protocol. The ethical solution, which the authors of the study chose, was to provide the highest standard of care as the placebo - in this case it was Truvada.

Since the treatment was actually even more effective than the existing standard of care, it was a home run success. Their conclusion is even stronger than it would be if they used a non pharmaceutical placebo, so in retrospect their decision was clearly correct.