Any fielder can allow a fly ball to drop to the ground...even if the infield fly rule is not in effect.
I agree...for most all other baseball or softball rule sets. But NSA does say that purposely allowing a catchable ball to drop, without touching it, is an intentionally dropped ball. I'm not aware of any other sanctioning body that rules it that way.
However, when the infield fly is in effect, it takes precedence over an intentionally dropped ball. Since the batter is already out on the infield fly a fielder should gain no advantage by intentionally dropping the ball. The runners aren't forced to advance, so if they take off it's just a dumb move on their part.
These two rules protect the runners from being put in jeopardy by the fielder's deceit. They do not protect the runners from their own stupidity.