It's a little more complicated situation than this. First off, since he housed them after the attack, he isn't complicit in the attacks themselves. Legally speaking, there's a difference between helping someone who plans on committing a crime, and helping someone on the run from a crime. There's no prison sentence for not turning yourself in and hiding from the police - just how criminal it is to help someone evade justice can be debated, but it's safe to say that it needn't be punished as much as helping someone actually carry out a crime.

Secondly, he was originally acquitted, getting 4 years (and compensation to victims) only on appeal. The reason for that is that it was very unclear how aware he was that the people he housed were the terrorists, as evidenced by some phone calls and testimonies. One side claims that you'd have to be impossibly dumb to not realise these guys were the terrorists, while the other shows in many ways how impossibly dumb the guy proved to be. In the end, it was hard to know how different he was to, say, a cashier who unknowingly let an wanted criminal buy food.