This is pretty funny. TurnItIn cites the following case law when explaining how their commercial use of people's work meets the standards of fair use. "materially impair the marketability of the work which is copied." Let's assume for a minute I'm running a term-paper mill and selling my term papers. The indexing of my papers in TurnItIn will destroy their marketability. It may not be ethical to sell term papers but I'm aware of no law against it.
A final problem I have with their page explaining copyright is it says nothing about end user licenses. I've distributed materials under licenses that forbids any derivative commercial use. Yet they will happily store my works in their system and espouse their position that they are completely within their rights.
There are some more interesting arguments in this 2001 response to an email. Including the statement in response to a query about a student who specifically denied the right to upload their paper to an www site. "[U]se of the student's paper would be deemed fair because rather than constituting infringement, the use prevents infringement of that paper from occurring." In other words we think our use should be OK so it is.