Scalability. What harm could one professor do by watching who's paying attention? Not all that much. With tech, one software update and hey, now you have a live time series of who's paying attention when- maybe scale attendance scores based on it. Oh, certain kinds of folks aren't recognized as well by the algorithm? Oh, turns out students are being trained to manipulate their facial expressions to seem attentive? Oh well.
Ooh, hey, we have this data on where everybody sits, too- that's handy, since it gives you more information about the social graph of the student body. Reliably sitting next to a Bad Person? Maybe you need some investigating.
And if a professor is caught mentioning subversive information in a lecture? Who was paying attention more than usual during that part? They seem interested, don't they? Might need to give them a talking to, too.