"BUt then why is she is jail? She's in jail because they know the man is a rapist and they're pissed off she said he wasn't
So in other words, the system is set up to let him off, isn't it"
You have to view them as two distinct cases. (once again this is from a legal theory P.O.V rather than any belief in how it should operate).
Case 1:
The man raped his wife. There is evidence against him in the form of a statement from his wife, however there is also a statement from his wife saying that he did not rape her.
She claims that this was made under duress. (we don't know either way what evidence there is to support this.)
The state prosecutors decided to drop the charges. The reason we are assuming,as it is the logical one on the bare facts we have, is that they have deemed the conviction unlikel due to the contradicting statements and the likelihood that they will not be able to meet the requisite burden of proof i.e. that the man raped his wife beyond reasonable doubt.
I can see the logic here as rape cases are already very difficult to prove to this burden of proof (for whatever reason, many people have their own beliefs on why this is so). I could also see the logic in bringing it to trial as there is a case to be argued for duress but if I were to be realistic I would not hold out hope for a conviction based on the way the husband has manipulated the system.
case 2: (this is the timeline as far as I can make out, if I'm wrong please correct me)
The woman makes the statement.
She is pressured to withdraw her statement.
The police inform her they will be proceeding anyway.
She is pressured to make a contradicting statement as dictinct from withdraw.
The police believe that she is making a statement she believes to be false with the intention of perverting the course of justice and she is charged.
Now as far as I can see it is not necessary that the police "know" the man is a rapist (they may believe it and know it in a personal capacity but this is a million miles away from "knowing" in a legal capacity) and as such the woman can be charged with making statement which were intended to pervert the course of justice i.e. gaining a conviction against the man for rape, without proving he's a rapist. Otherwise you could never convict someone for perverting the cours of justice if they were successful in their endeavours. e.g. lets say you are trying someone for perverting the course of justice because they made the conviction of a burglar impossible due to their acts. By the logic employed by some in this case, if you can't prove that the original person on trial for burglary is guilty then how can you claim their accomplice has stopped a burglar from being brought to justice?
As I keep saying this is merely the logic which can be used to explain it from a legal stand point in my opinion.Others may well think I've got the law completely wrong (and I may have,as I stressed earlier I've studied Irish law, not English) I don't personally think it justifies what has happened here, but it can explain how this has happened.