1. Turnitin cites fair use ("it's educational"), but one of the metrics of fair use is the extent of use (they use the whole thing), if it's used for profit (they are), and the affect of merchantability. As shady as it sounds, paper mills are not illegal, copyright infringement is. If a paper mill rejects your paper because it's been through turnitin, that hurts me as an author.
2. Another aspect of the monetization that really grinds my gears is that every paper I am forced to submit benefits turnitin by making their database larger and thus helping grant them a monopoly in this market. I am being forced to write articles for the monetary benefit of a company. If they're making money off my paper I want a cut.
3. Turnitin does not offer any way to remove one's documents from their database. If it was merely being used temporarily to compare for plagiarism against internet or book sources then that'd be reasonable. But they keep my papers forever and will continue to monetize them for years after the fact.
I understand the plight of educators that want to stop plagiarism, but forcing me to irrevocably offer royalty-free licenses for my work to a for-profit company that is then charging the school (which if it is public is being funded by my tax dollars) is just insane any way you slice it.
Unfortunately the courts sided with Turnitin on this issue back in 2007 and 2009 and somehow ruled that their for-profit use was fair use.