Suggesting that it is a question of copyright (because turnitin.com makes money on it) seems a bit like suggesting that paper recycling violates copyright. The work is not being used as the presentation of ideas or knowledge, based on their meaning, which can be copyrighted, but rather as a collection or words, the meaning of which are irrelevant.

As for the argument that use of the service treats students as guilty until proven innocent, the same applies throughout university. Can it be objected that we have monitors in exams because it assumes that students might cheat? Is the registrar's office unfair because it demands transcripts rather than simply believing what students say their grades were?

Yes it is a reaction to increasing class sizes. But it is a better reaction than the only other, stop asking for written assignments.

A career sessional.

True. Under the first sale doctrine, copyright does not apply to the recyclying of print texts. But, if I'm not mistaken, the first sale doctrine would not apply to a student electronic text. Since the student is the copyright holder, the teacher would need permission to transfer a copy and allow turnitin to use it.

or... maybe assign papers the likes of which are not available on the 'net?

or... maybe use a process approach with students so that we see the drafting at several stages of the writing?

or... maybe require more writing than just one hatchet paper over the semester so that we can see a student's writing style over several assignments?

or ... like assign writing that is to be done in class?

i think an either / or approach to this issue is fairly reductive.