I'm not surprised a syllabus is not a contract but I do think that most universities, when presented with a situation like this, have it in their interest to honor the syllabus.
Universities do not create syllabi, individual professors do for individual classes. Some universities don't even require a professor to give one to their students. I don't see how a syllabus is even really relevant here at all because (University wide) student honor/conduct codes and the like are what actually applies here.
Courts generally stay out of grading disputes (for good reason!)
"University faculties must have the widest range of discretion in making judgments as to the academic performance of students and their entitlement to promotion or graduation." - Board of Curators of the University of Missouri v. Horowitz (Supreme Court)
Not only that the probability of this going to court is zero - the university provided a replacement professor to assign grades instead of failing the class.
I also read about this in another paper this morning and it said that the university had assigned security to this class previously due to its behavior.
Generally, professors should try to find a balance that sets the syllabus to be difficult but still doable--difficult enough that students don't blow off the work, but also not so difficult as to frighten them away and cause them to drop in the first few weeks. Plus you can always move the grading scale down later which makes the students quite happy later in the term. Failing someone because they didn't muster up to this sort of syllabus is more "difficult" for the administration to roll over.
"Difficult" in the sense that admin will maybe, possibly sense a vague pang of internal conflict and disease (assuming they still have any sense of agency whatsoever) while rubberstamping the override.