TLDR; We don't determine sentencing lengths based on the crime alone, we determine sentencing lengths based on the expected duration required to ensure that society is safe.
With kids we expect to rehabilitate them faster.
The long version:
You may no see how it matters, but the reasoning is sound:
If you are going to be lenient on juveniles because they are not fully mature enough mentally to contemplate the full effect of their actions, why make certain actions exempt from contemplation?
> This does quite clearly show what the people in the justice system think is worse: an adult hacker vigilante is worse than a 16 yo rapist.
The criminal justice system does not evaluate the actions of a criminal in isolation, they largely take into account the intention behind the action (Look up Mens Rea).
You are just putting two outcomes next to each other for comparison, while the reality is that the intention and maturity of the mind behind those actions are what decides the judgment.
Should someone with the mental capacity of a 5 year old commit some violent crime, do you still think we should judge them as we do with a normal adult?
The ultimate problem with your argument is that the justice system is not about getting revenge on criminals. It's about making society safer.
When someone commits a crime we remove them from society for the period we think it will take to rehabilitate them[1], and ensure that they aren't able to harm society further during the time they are not rehabilitated.
The reasoning for juveniles to get a lower sentence is because we, collectively, think that it will be faster to rehabilitate a minor than an adult.
> I doubt you would get the same result if civilians were asked.
You'd be surprised: leave gender out of it and ask about "violent crime" and most parents would agree that the court should be more lenient on kids. That's because, as parents (and former kids) we know that kids make incredibly poor decisions.
Phrasing the question as "would you prefer your kids rapist to get off with a slap on the wrist" would get the response you want.
[1] Whether or not this is successful is irrelevant as we cannot know the success at the time of sentencing.