They may not employ clean-up crews for other sports besides football. If you have ever ventured through the lots late on Saturday after a football game, it is amazing the crews have started and pretty much have it cleaned up by Sunday morning. Also amazing how much debris/junk/garbage/tents/coolers/etc is left behind.
Now, granted, one group certainly wouldn't matter - especially if they cleaned up after themselves - but you also have to remember these are student parking lots that will get used the next day or two. This may be their reasoning (that and this is ND we are talking about)
That being said, I have "tailgated" for other sports if you loosely define tailgating as having coolers in the trunk and a group of us pregame drinking - no real food, grills, chairs, tents, tables, etc.
I might suggest - if truly interested in tailgating - the lot behind Innovation Park or Fairfield or Boy Scout Lot next to Parisi's would work - or just "tailgate" at O'Rourkes, Legends, or Rohrs (Morris Inn)