The golfer out of excitement threw the ball to the crowd, releasing ownership at that point.
The excited fan who caught it, excited as well to receive the ball.
No one at that point has any obligation to each other.
The Club has no ownership as well.
However: this being a historic ball, the fan determined that it should be returned, as is a good thing. Without it being autographed or somehow labeled in some way. It would be worthless now or down the road. No way to prove it.
The fan did the best thing, that is what good golfers and fans do. It's a gentlemans honor thing.
You am good folks, AM...