@joshguy just for the record I am agreeing with you that she should absolutely not work for them again.
However I do see why they did what they did. They knew she wasn't the criminal but all the evidence pointed to her and the alternative to what they did was letting her be executed for a crime she did not commit. At least by making her a criminal slave she is still alive and they have the time and resources needed to prove she is innocent and then set her free after wiping away the criminal charges. Plus they were able to give her to a slave merchant they trusted, i think, its been a while since i read the previous chapters.
Again, i totally agree she should not work for them again but if the alternative was death for a crime she didn't commit then.... well maybe this was the lesser of two evils situation.