The Big Ten is going to 9 game conference schedule and after the conference schedule starts they don't want any non-conference games (which means no game after September).
Of course it is ok if Michigan wants to schedule 1AA Delaware State on Oct 15 or so.... Like they did a few years ago....
Pac12 Big 12 SEC don't seems to have the same problem though.... Not surprisingly.
There is also this annoying scheduling habit of Michigan nearly always scheduling Central Eastern UMass etc both the week before and the week after our game. Some years we played a decent game before than MSU after (not easy) - so we had to get up three weeks Ina row - but Michigan - only one week.