This court case is actually about clickwrap agreements, not fair use. Though the courts spent a fair amount of time blathering about fair use, the clickwrap agreement was already decisive. The only reason that the fair use argument was even mentioned was because one of the plaintiffs "hacked" the system, using a password not intended for him and (apparently) not agreeing to the clickwrap agreement.
There's actually a really ugly line of reasoning in the cases about making contracts with minors. The minors were required to use Turnitin (sign over their copyright in their papers for free) by their schools. Since Turnitin didn't coerce them (the schools did), it's not a contract of adhesion. Since they received some consideration from it (the ability to graduate from their schools!), they cannot escape from the contract due to their being minors. SO: as a business owner, if you can get schools to require that students do X which benefits you, they have no hope of escaping from it legally, even if X is grossly unfair to the students.
Let's consider: how much would Stephen King's/Madonna's/Oprah Winfrey's/Barack Obama's student papers be worth today? We don't know, because no one required them to use an archiving service that could sell them later on. But in just a few years, some people from the Turnitin.com generation will be reaching superstardom, and Turnitin can sell copies of their student papers to all comers. I suspect that the value of selling specific student papers will far exceed all other revenues ever realized by Turnitin.