[umpduck11]

The intent of the rule is to eliminate the possibility of a "cheap" double (or even triple) play. Remember, the issue here is "intentionally dropped" line drive or fly ball.That requires a fielder to "touch" the ball, as in an attempt to catch the ball, then purposefully dumping it, or slapping it down, with runner(s) holding so as not to be doubled off after the catch.
The approved ruling tells you that the fielder MUST touch the ball for the call to be made, and that an IFF takes precedence. Touched or not, with the proper situation for an IFF, the umpire will have already called the batter out on the IFF.
Also, you only have to have a minimum of a runner a first to apply this rule. You need a minimum of first and second bases occupied to have an IFF situation.