It seems like either (1) Rivers was deceived, or (2) she was in some other way unaware that there was already an effective cure for syphilis which was not going to be given to the experimental subjects, or (3) the other options available to these people were so wretched that they were worse than having syphilis left untreated.

In cases 1 and 2, it doesn't really matter what we think of her calculations; if you're fed sufficiently wrong information then correct algorithms can lead you to terrible decisions. In case 3, maybe Rivers really didn't have anything better to do -- but only because other circumstances left the victims of this thing in an extraordinarily terrible position to begin with. (In much the same way as sawing off your own healthy left arm can be the best thing to do -- if someone is pointing a gun at your head and will definitely kill you if you don't. That doesn't say much about the merits of self-amputation in less ridiculous situations.)

I find #3 very implausible, for what it's worth.

(Now, if the statement were that Rivers believed that the benefits to the community outweighed the risks, and indeed the overt harm, to the subjects of the experiment, that would be more directly to the point. But that's not what the article says.)