[Note: I'm posting this as a second answer since it adds a new (and, I think, important) insight that's completely unrelated to my first answer.]
I think a key point to understand is that the value system of academia is different from the value system of the rest of society, and that that makes perfect sense. I don't mean that academia and everyone else disagree on what's moral behavior and what isn't -- I think by and large, in a qualitative sense, they agree on those things -- but they have quantitative disagreements on the extent to which certain behaviors are moral or not (and therefore how severely they should be punished).
Specifically, in academia, honesty and honest behavior (in a professional context) are much more prized than outside academia. That is because this type of honesty is essential to the mission of academia, which is to advance human knowledge. So, for a professor to cheat on his wife is seen by other academics as not good in exactly the same way, and to the same extent, as it would be seen by anyone else. But for a professor to copy a section of a paper written by someone else and publish it in his own paper without attribution, would be viewed by other academics much more severely than it would be viewed by most people outside academia, because it is not just "ordinary" dishonesty, it is a special kind of dishonesty that discredits and harms the entire profession and its mission.
Note that this is analogous to the situation with many other professions that have their own unique value systems and codes of conduct that are different from the rest of society. For example, lawyers care much more about confidentiality of their clients' information than other professionals, and that's why if you are a lawyer who disclosed some information about a client that you were not allowed to disclose, that would be viewed much more severely by your profession (and could lead to harsh punishments such as being disbarred and forbidden from practicing the law) than if you are just a random guy (even a lawyer) who was told a secret by a friend and told it to someone else without permission. The same violation of trust, which according to our normal moral code is just as bad in both cases, is interpreted completely differently according to the context in which it took place, since in the latter context a different value system and moral code would apply. (Similarly, doctors have their own unique codes and find certain behaviors unacceptable in a professional context that most people would not find very problematic. I could come up with some examples to drive home the point but this post is already getting a bit too long.)
To summarize, although in my first answer I argued that it's not necessarily the case that plagiarism is punished more severely than any other offenses, here I want to argue that even if it is punished more severely, that could be rational and based on the unique value system of academia, which holds certain values, and in particular professional honesty and integrity, as much more cherished and important than the rest of society does. When viewed in this way, I think this situation makes perfect sense and is precisely what you'd expect to happen.