But more interestingly, why would it be "highly unethical" to do so? They're practicing informed consent, so everyone who gets a shot knows that there's only a 50% chance that they got the real vaccine. And they know that the vaccine isn't guaranteed to work (quite the opposite, in fact, based on the track record of similar trials here in the States).
"There is no placebo group" vs "Half the sample population gets the treatment half of it doesn't"
The "doesn't" is the placebo group.
And since every single other HIV vaccine has failed, it's likely that this one will also - so it's not like they are withholding an effective treatment and acting "highly unethical".