Secession movements have a big chance of killing a ton of people and violating the civil rights of many more, so a 15 year sentence is very plausibly appropriate, in comparison to armed robbery.
So basically you're saying that somebody should be jailed for N years because maybe, in the future, someone else related with the same movement might use some level of violence to achieve... something we still don't know, in circumstances we still don't know?
Wow.
