A student wrote with a great question, that is, "What does plagiarism have to do with writing conventions?"
If you think about the definition of writing conventions I've been using with you, that is, a shared but arbitrary set of expectations governing communication within a discourse community and/or rhetorical situation, then you realize that the notion of conventions is pretty insidious.
There are conventions of behavior governing most author/audience interactions. There are conventions about the value of words and who owns them and the ideas the words encode. Words are tangible. At least, words are tangible in relation to something as intangible as an idea.
Conventions reflect and reproduce the beliefs of society. The notion of property ownership is an essential one to a working capitalist society. It might also be essential to a working democracy. In our society, we also have to value innovation and, hence, novelty; indeed, sometimes we place too much value on novelty and not enough value on experiences which aren't novel. (This last is just my opinion.) In any event, combine the social value placed on novelty and innovation with that placed on ownership and control of ideas, and you have most of the recipe you need to give an idea like plagiarism--stealing ideas, words, or the structure of texts--some pretty heavy weight.
Throw in one last factor, namely, the German university system. I won't go into the German university model. Suffice it to say, the model was adopted in Americia in the late 19th Century as a means of encouraging the production and circulation of the complex knowledge surrounding science and technology. One side effect was that most professors get rewarded by producing knowledge and by the reputation they bring to their universities. This is why teaching may take a back seat to research and writing at some universities (but not at UAT!). Combine the idea of the professional author with patents, copyright, and the way professors are rewarded in the current university, and you create the notion that one's ideas should be given proper homage, respect, and remuneration. Remember, in many respects, university education is meant to socialize the community's leaders and reproduce the institutions of society, including the university itself. Professor's value the knowledge they produce, so they expect everyone to value it equally.