Breh, that's the rule. Don't know what else you want to me to say
Edit, just noticed the question of where I got it from.
NFL overtime rules
APPROVED RULINGS
RULE 16-SUDDEN DEATH
A.R. 16.1 KICKOFF RETURN FOR TOUCHDOWN
Team B receives the opening kickoff and returns it for a touchdown.
Ruling: Game over. Team B wins.
A.R. 16.2 ONSIDE KICK
On the opening kickoff of overtime, Team A legally recovers the ball at the A41.
Ruling: A's ball, first-and-10 on A41. A kickoff is considered an opportunity to possess for the receiving team. Team B is considered to have had an opportunity to possess the ball.
A.R. 16.3 ONSIDE KICK
On the opening kickoff of overtime, Team A attempts an onside kick. A2 legally touches the ball at the A42 but muffs it and it is recovered by B1 at the A43.
Ruling: B's ball, first-and-10 on A43. Team A is not considered to have had an opportunity to possess the ball.