Hartley was axed from the 50-player named provisionally for this year's tournament in England after being suspended for a head-butt playing for Northampton in the Premiership semi-final, the latest in a long line of disciplinary issues.
"Dylan has had numerous occasions when it has happened," Johnson told the BBC on Saturday.
"I think Stuart made the right call. If that happens at a key moment it could cost everyone. So Hartley almost made the choice for him."
Johnson, who lifted the World Cup trophy in 2003, was England coach in the last World Cup in 2011 when his players were criticised for off-field behaviour and lost to France in the quarter-finals.
"It's not rocket science is it? Don't get into trouble," Johnson said.
"You have to know what the line is between right and wrong.
Ultimately when you go to a World Cup you have to be able to trust everyone.
"I think the players know where the line is and they know what they need to be doing or not doing."
England centre Manu Tuilagi was involved in controversy in 2011 when he jumped off a ferry in Auckland harbour and he will miss this year's tournament after pleading guilty to an assault charge.
"It's unnecessary," Johnson said. "He's a fantastic player but he's lost an opportunity and ultimately only got himself to blame."
Johnson believes the World Cup is wide open.
"I think a lot of teams will think it's their best chance in a long, long time to get to a final," he said. "So it's going to be very interesting what plays out.
"Playing in a World Cup the games are huge, the pressure is massive. It's who can deal with that."
(Reporting by Ed Osmond; editing by Amlan Chakraborty)
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