Great question. It's funny isn't it? Typically when people are caught committing fraud, they are forced to repay those they defrauded, and are likely to face prison time as well, but not so with paternity fraud. When it comes to paternity fraud, men who have proven they have been defrauded are often ordered by the courts to keep paying. It's crazy.
The reason of course is that unlike other types of fraud, paternity fraud is a type of fraud in which only males are the victim and in which only females benefit from this fraud. Since society is gynocentric, it is quite happy to treat paternity fraud differently than any other type of fraud.
This of course is not unique to paternity fraud. We see male victims of statuary rape forced to pay child support to the person who victimized them, while the person committing the crime goes unpunished. We have a violence against women act, despite the fact men are victims of violence more than men and desire the fact women instigate more domestic violence than men. It's common knowledge that women are less likely to be go to prison for any crime than men are and if they are, typically receive much lighter sentences. Society simply does not want to acknowledge males being victims, and wants to reward females, even if they are the ones committing the crime. It's just how it is.