7 years is not long for beheading, defiling, and cannibalizing a stranger on a bus
If 7 years is how long it took to be sure he wouldn't reoffend, why go longer? It would be a waste of taxpayer money, it would worsen his life for no benefit, it wouldn't solve anything for society, and it wouldn't even be useful as a deterrent because I'm pretty sure he wasn't thinking about ROI of crime vs. potential punishment when committing the crime considering he believed God was ordering him to do it, and he didn't deny having done it.
If mental health treatment were so far advanced that his problems could have been 100% cured within a month, I would have supported his being released after a month.
Why are we talking about benefits for a person that beheaded and ATE someone?
And also it's because he's still a person. A person who did a terrible thing while literally insane, but still a person.
It feels good, but it's like letting a drunk driver get back behind the wheel after they show remorse. The chances that they get in another DWI (and hurt someone) is high.
It's also why we have sex offender registries. The recidivism is high.