Now suppose your best bud 25 students is not the only one. Another professor teaches 9 hours with an average enrollment of 20 and another 9 hours with an enrollment of 15. The first generates 180 student credit hours and the other only 120. Remember, this is out of a fair share of 500 per year and the externality accumulates.
Do they think about it? I doubt it. Have you ever heard a law professor say "I just do not feel I am pulling my weight. I'd like to teach a bigger class."
I have framed this as choice but it may not always be. Evidently one of the ways to avoid to the fair share is to be awful in the classroom. So, you might be assigned to teach a potential large group of student but they do not enroll. Or, as happens in some schools, you assign the person to a large first year section, the students protest and the response is to reassign the teacher to something no one is required to take and very few do. It may not be a choice but it is an externality nonetheless.
Teaching is not the only place where professors are quick to let those they refer to a "colleagues" eat their dust. When do you want your classes to be? How about 10-11 MWF. Due to space and scheduling conflicts, not everyone can have that time and those days. Has a law professor every written to his or her dean "I've asked for and received the perfect schedule the last 5 years. I know that means others have not. Consequently, please determine my schedule after accommodating others." I did not think so.
There are many other examples of shifting costs to others. Here is another. Your school schedules class for the Wednesday before Thanksgiving. And each year students ask "Are you going to hold class on the day before Thanksgiving?" Odd question, you might think -- the schedule does not say it is a holiday. Eventually you learn that a fair number your colleagues cancel class that day and the pressure is on you to make it a clean sweep. Does the canceler ever think "Does this effect others?"
Then there are, of course, the make up artists. These are the folks who leave for a couple of weeks and then make up the classes (if they do at all) at semester's end. Here the externality is principally absorbed by the students but it is also not productive to try to teach students who have just had a marathon make up session. At my school we actually have a sanctioned program that requires people to miss class for two weeks. Yes, institutionalized externalities.
My hunch is that this goes on in most jobs but, in my view, law profs who talk about collegiality while producing externalities wouldn't know what collegiality meant if it bit them in the butt.