They figured they had Oswald dead-to-rights.
So they in fact were honest from the get-go.
They wanted to make sure there were no conspirators.
That's it.
But their messaging was badly written, at least in part.
Just bad enough to get the cultists all atwitter.
It was a very dishonest process. There were no dead-to-rights for Lee. J. Edgar Hoover said in a memo two days after John F. Kennedy's assassination that the public must be led to believe that Lee Harvey Oswald acted alone. You don't lead someone to believe something until all of the evidence is compiled and this memo was released mere weeks after the murder. They hadn't even yet come up with an explanation for the shot sequence and timing.
Katzenbach's memo says that "...it is important that all of the facts surrounding President Kennedy’s Assassination be made public in a way..." that he's the lone assassin. The "in a way" is another example of putting the cart before the horse.
There was no such thing as dead-to-rights. Oswald has absolutely no legal representation during the time he was in custody which is why he asked for representation at the press conference. It was his right to retain legal representation.
Some WC members did not agree fully with the WC's conclusions. It's a mess beyond all comprehension.