Because I would be abusing my authority if I said "yes". So you shouldn't even ask.
[Updated: see below]
This post is about teaching economics, though it could be about teaching anything else. It's also about allocating scarce resources between competing ends, which is the subject matter of economics.
I teach a big first year economics class. 330 students in the lecture, plus another couple of hundred watching over the internet or on cable TV. That's a big captive audience. And a lot of people would like the undivided attention of a big captive audience. So they ask me for my permission to speak to my class. They forget it's not really mine, except within limits.
I used to sometimes say "yes". Then I mostly stopped saying "yes", on pragmatic grounds, because there were so many requests, and I had to draw the line somewhere. Now I nearly always say "no", on grounds of principle. Maybe in future I shall start saying "no" less politely.
There are 3 hours of class per week, nominally (plus 1 hour tutorial with a TA). That's really 160 minutes, because students have 10 minutes between classes, and there are two classes per week.
Within certain limits, I am the one who decides how those 160 minutes get spent. I allocate those 160 minutes worth of resources -- my time, the students' time, the lecture theatre -- between me lecturing, the students asking questions, writing tests, etc.. I might listen to student input, but I still make the decision.
I allocate those 160 minutes of our resources because I have the legitimate authority to do so. You might say that I have the legitimate authority to act as central planner within this limited sphere because I know more about economics than my students, so I know best what is the best use of those resources in an economics class. But that's not where my authority ultimately lies. I have the legitimate authority to decide what we do in that 160 minutes because the students chose Carleton, Carleton chose me, and the students signed up for my economics class.
But they signed up for an economics class, so they can learn economics. And my legitimate authority extends only so far as using my best judgement to allocate those 160 minutes to try to help them best learn economics. My legitimate authority is limited. If I used my authority for some other purpose I would be abusing my authority. Even if I thought it would be a good thing for my students to spend some of those 160 minutes doing something else, that is not my call to make.
So I say "yes" to the students' economics society, because I think joining the economics society is a good way for students to learn more economics, and a presentation by the economics society is a good as well as a legitimate use of a couple of those 160 minutes. The students' commerce society is a borderline case, that I can maybe justify on the grounds that the two subjects are closely related. Maybe I should let someone make a short presentation to sell subscriptions to The Economist, on the grounds that it would help teach the students economics?
But that's about it. I have the legitimate authority to allocate those 160 minutes of my students' and my time to the teaching of economics. Within those limits I can try to use my judgement to do what I think is best. I do not have the legitimate authority to allocate those 160 minutes to anything whatsoever, even if I do think it would be a good way for the students to spend their time. I cannot pick and choose between what I think are "good causes" and what I don't. It's not my call to make. It's not my time I'm spending. The students' time doesn't belong to me. They signed up to learn economics, and that's what they are going to get. Even my time doesn't really belong to me during those 160 minutes. My time has a job to do.
So I say "no" to: student politicians; people offering summer jobs; psychology professors wanting to do experiments; people who want to warn against gambling; groups who want to talk about violence against women; religious groups who want converts; and lots of others I have thankfully forgotten.
They are asking me for my class time, and my students' time, which aren't mine to give how I want. They are asking me to abuse my authority. They shouldn't even ask.
I have sympathy for some of them asking though. If other politicians ask, and other professors say "yes", then a student politician who doesn't ask is at an unfair disadvantage. We should all say "no".
And if there were an imminent danger of fire in a crowded lecture theatre, then of course I would make an exception, and let the fire marshall speak to my class.
[Update: There are two ways to read this post. I think I should have put more emphasis on the second way. I'm going to do that now.
Economics is about the allocation of scarce resources between competing ends. And this post is about one particular example of allocating scarce resources (160 minutes of class time) between competing ends (such as letting someone else speak to my class). Clearly, the question in the example matters. That's the first way of reading this post. But maybe this particular example is simply one example of a more general question, and we can learn something from this example about other questions that have nothing to do with teaching. That's the second way of reading this post.
At the most basic level, the economic question is: "how much land produces apples and how much land produces bananas?" And an answer to that question is something like: "6 acres in apples and 4 acres in bananas."
At the next level, the economic question is "what method do we use to arrive at an answer to that basic question?" And an answer to that question would be: "use the market": or "a central planner decides"; or "we do what our forefathers did".
As Coase and other economists have noticed, what we think of as a "market economy" in fact has lots of pockets of central planning as well as markets. Firms are one example. Maybe the household is another. My classroom is a third.
In talking about that third example, I have found I cannot talk about the method by which scarce resources are allocated between competing ends without introducing the concept of "legitimate authority", which puts important limits on my role as central planner.
Maybe that concept of "legitimate authority" is an economic concept that is equally relevant in other examples of allocating scarce resources between competing ends. Like inside firms. There are things a manager may not do. Like use the firm's resources for his own ends. Or give the firm's resources to charity. (Unless the owners have authorised him to do so.)]