It was a bad call and the game is under protest, so it is hard to fault the fans for getting little rowdy.
The problem was they were hitting some of their own fans which included children.
Can’t have that.
Here is the rule for the in-field fly rule.
The infield fly rule is a rule in baseball intended to prevent infielders from intentionally dropping pop-ups in order to turn double plays (or triple plays). Without this rule, a defense could easily turn a pop fly into a double play when there are runners at first and second base. If the runners stay near their bases to tag up, the defense could let the ball drop, throw to third base and then to second, for a force-out at each base. If any of the runners stray too far from their bases, the defense could catch the pop-up, and double-off any runner that failed to tag up.
When the rule is invoked, the batter is out (and all force plays removed) regardless of whether the ball is caught, thus negating the possibility for multiple outs.
The ball was in shallow left field so it seemed odd that it would be called in this situation.
Here is video of the play.
The Braves protest has been denied.