Because it is a tick mark for their quota... I mean mission objective. It's an arrest and conviction. Freakonomics at its worst. The criminal justice system needs fed and it doesn't care about the people it consumes.

This is a step in that direction. It's like this. It it is on the books as a crime; some sheriffs and DAs will charge the child. That is just how it is. And let's not forget how badly people in general are stigmatized once they step in the back of a squad car. To the cops on the beat, that child is just another child offender. The cops on patrol treat them the same as they would a kid vandalizing a school and selling pot. By decriminalizing the underage victim, the child leaves with child services instead of the PD. It is just a better way to start the process of removing the child from the situation.

further reading on the subject that I haven't fully read :

http://www.unh.edu/ccrc/pdf/CV62Bulletin.pdf

https://rights4girls.org/wp-content/uploads/r4g/2015/02/JJ-Trafficking-July-2016.pdf

https://fas.org/sgp/crs/misc/R43677.pdf

https://www.ovc.gov/publications/factshts/cevcjr.htm

Child sex workers are usually forced into it either through homelessness or through coercion. What has been happening is the children were rescued from their pimps... just to be charged with prostitution. So they get a double whammy of being sexually abused AND having a criminal record for sex work. Rather than find these kids a good home and therapy, they are put into jail cells and turned into institutionalized criminals. The new changes lets the child off, but not the John. The John still gets busted for raping a child.