One of the biggest problems with the reaction here is that we expect victims to be heroes. The corollary of that is that if Cain (or Magness, Goucher, etc.) is not a hero, then she's not a victim. And so the discussion becomes a referendum on Cain, not on Salazar and the NOP.
It doesn't matter if Cain's a good person. Maybe she's a wonderful person; maybe she's a horrible person who just wants to get back at Salazar for not letting her rejoin NOP. (Obviously I would hope for the former.)
It doesn't really matter too much. What matters is that Salazar, by many different people and for many different reasons, has been accused of being a scumbag. Disbelieving this doesn't just require disbelieving Cain, but also Cam Levins, for instance, and many other people. And it requires another explanation for the weirdness that we've always known was there with Cain and the NOP.
If Mary Cain were the only person who has said something this negative about Salazar, then it would be her words against his, and it would be perfectly reasonable to question her motives and her character. But they're simply not relevant in a world where many others have said the same thing, and none of the accusations are inconsistent with who we already think Salazar is.
It's perfectly reasonable to say something like "Salazar's 4-year ban was too harsh given the relatively meager doping violations that USADA was able to prove". But to assert that Salazar isn't a major pox on the sport requires that you dismiss or disprove: the proven doping violations (and suspicion of more), the grey area behavior, the psychological manipulation of athletes including publicly berating them and focusing disproportionately on their weight, the hostility to the media, trying to get personal information of Letsrun posters, and intimidating rival athletes and coaches while playing politics with the rules of the sport (i.e. the Gabe Grunewald DQ).
In this ecosystem, the force isn't with any one accusation, but with the sum total of all of them. That's why it isn't about Mary Cain: it's about Alberto Salazar and the things that he's done.