There's also Lester's boy-toy Dr. Coleman to consider. Maybe he used his connections to shield Lester from any treatment that she didn't want. Or perhaps she did get treatment, and it just didn't work
Then again: Before Lester swapped with Kirk, had she been convicted of any crimes at that point? Obviously she killed the rest of her team, but nobody knew
about it at that point. She can't get treatment for a crime that the Federation doesn't even know about, let alone actually convict her for.