I am involved with corporate events. We regularly spend $150,000 for a week's worth of Wifi & bandwidth for a conference of 2000-3000 attendees. The bulk of that is on setup, not on the length of the conference.
Could the attendees bring their own Wifi hot spots? Sure! Would that quickly overwhelm the capacity of the local cell towers? Yup. So the only real option is for us to provide the connectivity.
At corporate events there are charges for the attendee to attend the conference - either it is internal and per employee, or people sign up and pay. In either case, we simply build the cost of the connectivity in to the overall package.
I suspect the Presidential Debates do NOT have significant cover charges for media to attend. Thus they need to recover the infrastructure investment somehow. And remember, the bulk of the expense is on the setup - not on the length of the event.