Jim Garrison claims he asked for a preliminary hearing on the Clay Shaw prosecution because of fairness. Here is what he says in his book, On The Trail of The Assassins: (page 151)
"In this case I made the motion out of fairness to Shaw because of the extraordinary seriousness of the charge. My application requested that a panel of three judges then determine whether the charges should be dismissed or Shaw held over for trial. This was the first time in the history of Louisiana that such a motion ever was filed by the prosecution on behalf of the defendant."
Here is an excerpt of an internal Life Magazine memo written by Sam Angeloff: (from the papers of Richard Billings at Georgetown University)
There you have it - right from James Alcock, second-in-command to Jim Garrison - the reason the prosecution asked for a preliminary hearing was to preempt the defense and ensure that friendly judges were on the panel. This was during the period when Life Magazine was working with Garrison on the investigation.
Here is what Clay Shaw wrote in his journal about the reasons for a preliminary hearing:
And, as we know, the only evidence presented against Shaw, related to the assassination, at the preliminary hearing was Perry Russo's false memory of a conspiracy meeting at David Ferrie's apartment - a memory that was implanted through the use of hypnotism and sodium pentothal.