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.
Or (4), she was led to believe, either explicitly or implicitly, that her career and livelihood would be in jeopardy if she did not participate—thus motivating her to subconsciously sabotage her own utility calculations and then convince herself that the sabotaged calculations were valid.
Or (4), she was led to believe, either explicitly or implicitly, that her career and livelihood would be in jeopardy if she did not participate—thus motivating her to subconsciously sabotage her own utility calculations and then convince herself that the sabotaged calculations were valid.