Furthermore, most academic institutions of higher learning maintain honor codes that must be upheld by all students as a condition of their matriculation at the institution.

Academic honor codes are formal representations of the student's acceptance of the terms established by the institution governing all elements of academic and social conduct associated with the pursuit of the degrees awarded by the institution. Therefore, by definition, every form of academic dishonesty (including but not limited to plagiarism) is necessarily a fundamental violation of the institution's honor code.

Plagiarism is Unfair to Honest Students:

Generally, the most common grading scheme used by college-level instructors incorporates some form of the bell curve, according to which the relative performance of all students within a given course is factored into the grading scale used...

The bell curve allows instructors to adjust grading to reflect the appropriateness of the difficulty of the course material as well as the instructor's ability to transmit the course material effectively.
Therefore, plagiarism (and any other form of academic dishonesty or cheating) throws of the purpose of the bell curve by artificially increasing the average scores of all students, including those who executed written assignments completely within the requirements of legitimate academic research and writing. This is fundamentally unfair to honest students because it lowers their grades, in addition to allowing students who cheated to achieve comparable grades they do not deserve. Finally, since the bell curve provides an important feedback mechanism for instructors, plagiarism that improperly increases any student's grades provides erroneous information to the instructor to the potential detriment of future classes taught by the same instructor.

Conclusion:

Plagiarism is a form of academic dishonesty that involves the misrepresentation of written work as the product of one's own legitimate academic research and original writing. It undermines the very purpose of education for the plagiarizing student, in addition to unfairly diluting the performance of students whose academic work represents their honest academic efforts; it may also harm the academic institution and employers.