I don't think it's entirely true to say that the only reason he's been jailed is retribution. I think there is likely also an element of deterrent involved here, and I do believe that sending out a message to any other would-be parliamentary-disruptors is not a bad thing. It's certainly far more worthy than retribution. That said, I do agree with you that community service would have been a more appropriate sentence.

(Whether the criminal justice system actually works as a deterrent is another matter and is up for debate, but I believe that that is generally considered to be one of its goals, and is definitely something I'd consider more worthy than retribution, which I agree should not be a factor at all.)