If the instructor collects interim evidence of students' progress on the assignment, it is unlikely that the purpose of the assignment will be undermined by downloadable term papers. It would generally be more difficult for a student to construct plausible interim products that might lead to a downloaded term paper than to write a term paper. In any case, anecdotal evidence suggests that plagiarism is rarely a part of a long-term plan. Most students who plagiarize decide to hand in another's work as their own in a panic at the last moment. If students have been turning in interim evidence of their work throuoghout the semester, they are less likely to panic at the last moment.
Some assignments are relatively easy to fulfill with plagiarism.
Write a paper summarizing what you learn by reading at least five reports of empirical research. Relate your summary to one of the theories we have discussed. There are, by conservative estimate, a zillion such papers floating around out there. Other assignments may or may not be easy to fulfill with plagiarism, but create conditions ripe for plagiarism. We are often tempted to ask students to write substantial papers which they won't be able to begin until later in the semester, because we will not have discussed essential ideas until then.