The only way to enforce it would be for the NCAA to step in an make this a rule, not just Iowa State. And I don't believe that is tips the scales in favor of the school or program at all. It simply states that you get a year paid for each year you see the field. If you reshirt, then 90% of players need the 5th year, while playing football to graduate anyway. It does not penalize the player IF he declares to the staff by say, Septemeber 1st of his junior year, if he plans to graduate early. If he has the accountability to inform the staff then he just got 4 years of classes paid with 3 years of playing time. That's a pretty good deal. The athlete would only be penalized if he did NOT declare his junior year, that he would be leaving early, but does anyway without notice. In that case, the program/staff is at a huge disadvantage because they thought the kid was staying, but hangs it up with the prime recruiting period already past, not to mention the cost to the program to go out and recruit a kid to replace the one that quit. The kid would have received 4 years paid tuition, but only played 3.
It all hinges on whether you inform the staff of your true intentions if it looks like you want to graduate before the 5th year. If you do, no penalty.