I doubt "externalized cost" is the real reason professors ban laptops because if that were the case, a simpler solution would have been used: reserving the first few rows for the minority of students to claim to be distracted by others' laptops. My hunch though is that professors use these occassional student complaint as a front for their own insecurity of being ignored when they talk (which bad professors are most likely to be susceptible to).
In my experience the laptop-banning professors are also the ones to practice other the other common destructive behaviors: 1) Deducting grades for missing class, 2) Dropping exam hints in class to encourage attendance, 4) Pop quizzes, 3) Not posting notes digitally. A professor doesn't need to be a charismatic entertainer to teach effectively, but they do need to somehow make their lecture a better use of students time than just reading the book or notes. Trying to make it HARDER to learn using any method that doesn't make them feel good (laptops in class, staying at home and reading the notes and book) may help their ego, but does not help students learn.
shrugs Again, my example has none of those, and my own classes generally don't either unless class participation is a graded component of the class (some classes are legitimately discussion-oriented classes).
I'm not defending bad teaching; I've experienced plenty on both sides of it. But I don't think it's fair to conclude that laptop banning is always coming from a bad motivation. As I said - it's a practice I've seen adopted by some of the best teachers I know, in addition to one that seems like it could be a crutch for some of the worst.
In my experience the laptop-banning professors are also the ones to practice other the other common destructive behaviors: 1) Deducting grades for missing class, 2) Dropping exam hints in class to encourage attendance, 4) Pop quizzes, 3) Not posting notes digitally. A professor doesn't need to be a charismatic entertainer to teach effectively, but they do need to somehow make their lecture a better use of students time than just reading the book or notes. Trying to make it HARDER to learn using any method that doesn't make them feel good (laptops in class, staying at home and reading the notes and book) may help their ego, but does not help students learn.