Don't you know.... it's common knowledge JFK shot himself...
You see due to a mistake with their time drive, Dave Lister, Kryton, Arnold Rimmer and the Cat accidentally interupted Oswald in his deed.
JFK's living ment that he was exposed as a mafia puppet amongst other things and he was imprisoned. Lister and crew, being unable to get back due to the now non-existance of their space ship Starbug, had to get JFK out of prison, then convinced him to come back to Dallas to seal his legacy as a great president...
If you do not get my post, you FAIL.