Jay has heavy influence on the performers. Rhianna credited Jay for getting her the show. Nobody pushed back on this at the time. Usher literally just said Jay is the reason he got the show. There are tons of articles on this as well like the one below:
Usher was Jay-Z’s Super Bowl call, all the way
50 cent also claims Eminem had to threaten not to perform cause Jay was blocking 50 from performing during the superbowl. So while Jay is not the sole decision maker.. The cities will only get to pick from whatever Jay's approved short list is and Jay definitely has influence and can convince them who to pick. So lets not pretend Jay doesn't have the biggest influence on who performs.