She had a kid, and wasn't raising her very well. She was sad. She was a mess. But she wasn't a danger to anyone. A stiff sentence, which was likely given that she was a repeat offender, wasn't going to make her stop using drugs.
Users come out and usually use again.
She would probably keep using in prison. Lots of stuff gets smuggled in. There is nothing about prison that makes people suddenly have a conversion experience. You and I might think the fear of jail would make people stop. I don't care whether she respects the law.