Trying to secede in a country where secession is illegal, is... you know... a crime.
You can secede in federalist countries, but most republics prohibit secession -- since most of their constitutions are based on France's model.
So, a prison sentence was pretty clear for most political observers. The only way they wouldn't get prison terms was if the secession succeeded, which it did not.
Technically speaking, I'm surprised they weren't charged with treason -- the de-facto charge in such cases. Good thing the EU requires members to abandon the death penalty to join. Most high-level treason charges usually end up with a noose around someone's neck.
Also, please downvote me. Actual statement of facts aren't welcomed here.