that's a well-constructed PR myth. johnny cash never went to prison, and he readily admits it.
PR myth? Fuck whatever. He was in jail. Prison suggests that one spends an extended stretch incarcerated. Cash spent a few days for drugs, stealing, picking flowers or whatever. I don't think anyone built it up to be a PR move, I think people drew their own conclusions, it wasn't some masterful scheme by Cash.
And yes, I do resent the fact that Metallica tried to go bad ass by having their video at San Quentin. Those femme-bots wouldn't last two seconds in the joint. Boy, that guitarists hair sure looks purty. How about Lars? James? Dudes are getting fucking old. They have about as much bad ass credibility as Dr. Phil. These guys need to give it up. Johnny Cash could actually back up playing at San Quentin, because he is, and forever shall be the coolest and bad ass artist EVER.
Johnny Cash is the undisputed KING of music.
And did you see the lack of reaction Metalica got in that video? Those inmates weren't feeling it.