"It's not like they couldn't replace them in a heartbeat. "
The rumors that I have heard were that if they tried to kick them out it could get tied up in the courts for awhile. I don't know how true that would be because if that is how Penn State would handle it that would look even worse for their image. If the Big Ten was to decide to kick them out Penn State could start to clean up it's image by maning up and excepting it.
The other question is, if you kick them out does that end their football program? What conference would pick them up? Also would you allow players to transfer out without having to sit out a year? I would allow players to leave unpunished, but this is the NCAA they try to act tough all the time.