Unless you are talking about a different rule than the one below, please post a source. The following rule comes up when I google the rule you stated and does not state what you said.
"Unless relocated by penalty, the kicking team’s restraining line on a kickoff is the 30 yard line on a six-man field and the 20 yard line for a free kick after safety. The receiving team’s free kick restraining line is 15 yards from the point of kickoff. There is no requirement to have a minimum number of players on either side of the kicker. If the result of a Free Kick is a touchback for Team B, and the placement of the ball is not relocated by penalty, the ball will next be put into play from Team B’s 20 yard line.
The ball must travel 15 yards on a kickoff (or place kick/punt after a safety) or be touched by the receiving team before members of the kicking team are eligible to touch it."
