Nasty, here is some info on why Purdue has a bye week on Sept 7.
Purdue Scheduling
In short, they had an opponent lined up for Sept 7 but something changed and they were going to be forced to have a bye week 1, so they moved their Sept 7 game to Aug 31, thus creating the bye before ND. As a Purdue coach I'd welcome a full game against a weaker opponent to see what I have as a team and then make adjustments during the bye week for my big opponent ND. Some folks find a way to solve problems and stay positive while others find the ways to fail and bring forth negativity.
Purdue did what they had too, and ND doesn't give a crap what Purdue does.
I've also noticed a trend (you mentioned it- 7 opponents had a bye week before ND) that ND opponents like the bye before our game, not just for schematic advantage but the campus and the city prepare for what is likely their biggest home game in several years. They want to capitalize on all that is ND visiting.