If I were on his jury, I would vote for nullification.
Actually, all he is doing is educating people about the legal process. I don't see anything wrong with that, even on government property. As long as he isn't targeting people with jury notifications and making it available to the public it is freedom of speech. The only issue is the uneven application of the law from one trial to another...
And yes, I think there are way too many people in prison that shouldn't be there, and way too many that get off easy.