Jim Garrison was a terrible -and from reports both malicious, cruel, and often violating due process- DA. The case against Shaw was for the most part made up. There is no connection between Shaw and the Kennedy assassination beyond a witness who turned out to be a known con and who failed the lie detector and admitted he was lying, and Garrison's own imagination. The worst part is even after Garrison's witness admitted he lied, Garrison went on with the trial anyway.
The reason the court decision let Shaw off the hook was because Garrison bs'd everything. Garrison was a liar and a blowhard, and thanks to Oliver Stone (whose movie "JFK" is based on the book of a man who thinks Communism was a conspiracy created by wealthy businessmen and who threw in every crazed conspiracy into that book) Shaw, who was a man who contributed greatly to his community, has his name besmirched which frankly pisses me off greatly that Oliver Stone did that.
Sane people like me have been trying to debunk Oliver Stone's populist make-believe for quite a long time now whenever someone brings it up.
If you would like an ample debunking, here it is:
http://www.jfk-online.com/jfkmovie.html