Libraries, bus stations, shopping malls, this list goes on. How many places could you not walk around in carrying just about any size package and not arouse suspicion as long as you're wearing brown coveralls and carrying a clipboard?
While I understand that there are many places where someone with a backpack wouldn't arouse much interest, terrorist carrying out an attack at a large university would never be detected until after the fact. I have two children attending one of the largest public universities in the country and I've seen the spectrum of people there. There could not possibly be enough security to keep tabs on all the suspicious looking, backpack clad persons roaming around that campus.