These allegations were made a while ago. The university has already completed an independent 3rd party investigation that generally agreed with the allegations and that these things occurred.
To your point, they were unable to determine if Fitz knew these things were happening. That said, as a head football coach, your ignorance to these activities in your locker room does not absolve you of wrongdoing. You should be aware of things like this going on. Further, plenty of data points that point to him actually knowing, at least in part.
Finally, due process and innocent until proven guilty are legal concepts in a court room, not in an employment environment. If you, as the CEO of an organization, allow these things to occur on your watch, that is easily reason for extermination.