I-Am-Zim
Captain
Yet he apparently destroyed an armada at a Klingon prison planet for no reason at all.
Apparently so. Maybe he felt like blowin' sumthin' up. And the distress call came from a prison planet. That doesn't necessarily mean that the armada was destroyed at the prison planet. Maybe the prison detected the attack and sent the call.
Also, the attack on the Kelvin took place on the edge of Klingon space. Maybe Spock came through in a location close to Nero's original entry. After Nero and his gang roamed the galaxy twiddling their thumbs for 25 years, they had to go back to the edge of Klingon space to get Spock. They may possibly have passed by a "Klingon prison planet" and had to push through an armada of Klingons on patrol at the border. There are all kinds of explanations for that. Few of them include Nero being held in a prison. Especially since there was absolutely no mention of it in the movie.