Looking at the all the data from last night, I think a couple things are clear.
The NCAA and the Big 10 have significantly more information than is publicly know. That could come from a variety of sources such as former employees that have insight into the sign stealing (Gattis), peripheral players (CMU coach staff), the "investigative firm" having way more information than what was leaked to the Washington Post and/or anyone that has some insight that dislikes Harbaugh (a cohort that in growing exponentially). When the investigation is complete, I think the evidence will be massive.
I also think that Pettiti and the Big 10 felt they were giving Michigan an off ramp. They thought if they showed the evidence available now, Michigan would see the advantages of a Harbaugh suspension during some portion of the remaining regular season. Those advantages would be reduced impact on existing players (no post season ban, still NC possibilities) and by self imposing, any additional information on the details of scandal would be greatly reduced to revelations by the news media. For the Big 10, it would overt open warfare between their members, mitigate partially the competitive advantage Michigan enjoyed and they could wait on the NCAA to wack Michigan in the off season to complete the process. Understand, I am not endorsing the Big 10's thinking. I am ready to buy the salt for the plowing under of Michigan stadium. I just laying out a plausible explanation of their thinking.
It is also reported that the Big 10 brought some very big sticks with the carrots they offered. It sounds like a post season ban is among them.