It is unethical to purchase homework answers because it's a falsification of source material. Buying work means that the student has consulted a primary source but hasn't given correct credit or citation to that source. Rather, the student is passing the work off as her own, which is falsification.
There are various types of academic dishonesty, including cheating; fabrication, falsification and forgery; multiple submission; plagiarism; and complicity. Purchasing a homework assignment falls under the category of falsification; however, if the assignment happens to be a paper, then that also falls under the category of plagiarism.
Homework and writing assignments are given to develop and assess a student's ability. Doing the work to search out and purchase an assignment doesn't allow educators to gauge a student's strengths and weaknesses. In addition, it denies the student the opportunity to complete the learning objective set forth for the assignment.
It is important that students understand the varying types of academic dishonesty and the consequences thereof. Universities have little to no tolerance for academic dishonesty, and students receive, at best, just a zero credit for the offending assignment with expulsion being the worst outcome. In addition, these offenses often stay on a student's academic record, preventing her from enrolling in another college or university.