Why does the infield fly rule only apply if runners are on 1st and 2nd?

Question

The other day something prompted me to review the infield fly rule, which says (emphasis mine):

An infield fly is any fair fly ball (not including a line drive or a bunt) which can be caught by an infielder with ordinary effort when first and second or first, second and third base are occupied, before two men are out. The rule is in place to protect against a team allowing a shallow fly ball to drop in with the intention of causing a force play at second and third or second, third and home. Otherwise, the team would be able to force out baserunners who had stayed put on a routine fly ball.

In these situations, the umpire will declare "infield fly" for the benefit of the baserunners as soon as it is apparent that the fly ball qualifies as an infield fly. The batter is out even if the ball is not caught, and the baserunners can advance at their own risk. If the ball is caught, the baserunners can attempt to advance as they would on a typical ball caught in the air.

The question I have is why is the rule applicable when first and second are occupied and not just any first?

This seems to me like this is ripe for a defense to abuse in order to almost assuredly get 2 outs by letting the ball drop while the runner on 1st waits, thereby creating an easy double play at first and second.

Is this assumption wrong perhaps due to the time for the fly ball to fall, the batter will have ample time to run it out and reach first, thus, the defense is simply in a state of being able to pick which runner then want to have on first?

Is this because presumably due to the fly ball's time to fall, the batter will have more than enough time to run it out and reach 1st? This question discusses the very subjective nature of the rule, but it seems odd to me to not extend that subjectivity to when a situation occurs that could potentially become a free double play, even moreso when the rule's stated goal is to protect baserunners who stay put on a routine fly ball.

Answer 1

why is the rule applicable when first and second are occupied and not just any first?

The baserunners can't easily advance on a fly ball. If there are 2 or more such runners that are potentially forceable, then the defense can easily manufacture a double-play by intentionally dropping the ball in such cases. This has been deemed to be something to prevent. With just a runner at first, a double-play isn't simple.

Is this assumption wrong perhaps due to the time for the fly ball to fall, the batter will have ample time to run it out and reach first, thus, the defense is simply in a state of being able to pick which runner then want to have on first?

Exactly correct. The expectation is that the runner should be able to make it to first if the fly ball is dropped, so that a double play can't be easily manufactured.

The infield fly rule removes an edge case that is deemed to be too powerful for the defense. But it does so by removing the need for the defense or the batter to execute. If at all possible, we would prefer that these players be rewarded for execution.

So while there may be an occasional case where you could get a hustling batter on a dropped fly, it's not common, and the downside of stopping the play seems to not be warranted.