Students often seemed puzzled that plagiarism is such a big deal. Why does a cribbed paper make professors so angry? I cannot speak for everybody, so I will give an answer for myself. However, I imagine that my reasons are pretty widely shared among my colleagues.
First, let me note that students don't do many things to make me angry. I didn't even get mad the time the guy came up to me after class and asked "Hey, I'm going to be out of town the next three weeks. Are you going to be talking about anything important?" I don't get mad when students disappear for two months and then show up with a flimsy excuse. I didn't get mad the time the student was scheduled all term to make a class presentation on a given day and then skipped out at the last minute because his grandma visited. I do get mad when students plagiarize. Very. A student once whined that he did not think he deserved to fail my course for plagiarizing on just one paper. I responded that I did not think he deserved that punishment either but they don't allow tarring and feathering anymore.
Here's why plagiarism makes me so angry:
1) Plagiarism insults my intelligence. There is a scene in The Godfather where Al Pacino, playing the part of Michael Corleone, confronts a turncoat. He says "Don't tell me that you are innocent. That insults my intelligence and makes me very angry." That is precisely my reaction when I catch someone plagiarizing.
Plagiarists believe that I cannot tell the difference between their writing and the writing of an expert. When they copy an expert's work and present it as their own, they clearly think that I am too dumb to notice. Ah, but I do. There is a world of difference between the writing of a beginner, however bright, and the work of someone who really knows the topic. The difference is like night and day to anyone who has been teaching at the college level for over thirty years, as I have. Or maybe the plagiarist thinks that I am too stupid to track down the sources they copied. But if they can find them, I can find them. And I do.
2) Plagiarism shows contempt for other students. Most students work hard for their grades. They burn the midnight oil, balancing jobs, kids, spouses, and the thousand-and-one other demands on everybody's time. They come to class and participate, though after a full day they would love to take a nap. They act responsibly, though it is often a strain. When they make a good grade, they have truly earned it.
The plagiarist doesn't work hard and sacrifice. Plagiarists let others do the hard work and they just copy it and get the reward. If they get by with their plagiarism, they get good grades just the same as those who deserve them. Cheating has many advantages over honesty. It has so many advantages that cheating might just become a habit. If plagiarists can rationalize cheating on their grades, they won't have too much trouble excusing themselves for cheating on spouses, employers, or the IRS. They might just cheat their way into divorce, unemployment, or jail.
3) Plagiarism is theft. When you take someone else's academic work and present it as your own, you are stealing from that person. And it is a particularly odious kind of stealing. Believe it or not, academic work is hard. You have to spend long years learning abstruse things and mastering difficult skills. You are expected to publish, and every published work is like the tiny tip of an iceberg, with a vast bulk of reading, thinking, and painstaking composition below the surface. When, after years of preparation, delayed gratification, and intense, focused work, you do produce some publications, what is your reward? Money? Hah! Your reward is the honor of others -- scholars and students -- who read your work and credit you for teaching them something and making their intellectual lives richer.
Those who steal your intellectual productions for their own unearned benefit steal something far more valuable than one who merely steals your wide-screen TV. The plagiarist steals your dedication, your sacrifice, your creativity, your intelligence, your passion, and your love and inspiration -- not to mention your sweat and tears. And why do they treat you with such contempt? Because they are too lazy or apathetic to do the work for themselves. They do not even have the excuse of poverty or drug addiction. They do it because they just can't be bothered.
4) Finally, and worst of all, those who plagiarize debase the whole academic process. The pursuit of knowledge is one of the highest and purest of human goods. At rock bottom, and for all their faults, universities are dedicated to the discovery and transmission of knowledge. Aristotle was right: Learning is part of the basic purpose of human life. Further, knowledge is intrinsically good, good for its own sake, whether it produces any material gain or not. The pursuit of knowledge is therefore a high, noble, and beautiful ideal.
The plagiarist spits in the eye of that ideal and drags it through the mud. For the plagiarist, academic requirements are just a hoop to jump through or an impediment in the way. Being required to think and write for yourself is absolutely essential for the pursuit of knowledge. But plagiarists aren't interested in knowledge. What they want is to spend four years partying and then get that piece of paper that will let them get on with their lives. So, if the choice is to write a paper or have a big party weekend, what does the plagiarist do? Hey, it's a no-brainer! Just lift the paper from the Internet! That professor will never know the difference! Jeez, what makes him so uptight anyway?