OK, still an extremely dumb concept. Can you imagine if offensive line walked around five wide refusing to move all the time because they were taught not to "break rank". You're a band. Your task of playing during the game is over. There is no need to be "in rank" anymore.
It's really not though. Imagine you are in the ISU marching band marching to or from Kinnick Stadium. You are required to march in step and play your instrument half the time. Do you think it's a good idea to just let a Hawkeye fan walk right in the middle of your formation? Nothing good can come from that.
Even when they (the ISU band) are marching to and from Jack Trice they don't let people into their ranks. It's tradition for marching bands to march as a parade to and from the stadium for games. This is not just a group of people walking around. The Cyclone fans at Jack Trice don't have anything harmful in mind, but they may be trying to screw around with people and make it harder for them to march to the stadium or at best are just clueless and are holding up the band and splitting it apart during the march to the stadium. This doesn't apply just to hostile environments.
I have absolutely no problem with the Iowa marching band trying to close its ranks and keeping people out. That's probably the one good thing they did that day.