If they're eligible the bcs sends an invite. They don't have to sell the second quarter of those laughers, they just want all of the tickets purchased .those first two teams are not relevant to the bowls who would be looking at ND. Iowa is headed for orlando, and a 3 loss sc squad would also be in a different realm from nd.
Michigan is a very rare abomination that would be pumped for a terrible bowl game, unlike most years. If they weren't tied to an automatic bowl selection, which is likely with that record, they could always play nd in the bowl. It would be more interesting than a soeful bcs team or average mid major.
The bowls that pick 6-6 squads generally get garbage. A disinterested nd fanbase will still fill more seats than the typical participants of the papa johns bowl.