It seems like the Stanford service is trying to justify themselves by being an academic study and limiting to 500 concurrent participants... but it's telling they don't list any legal justification on the site, and merely generically claim:
> Stanford respects the intellectual property rights of others. If you believe your copyright has been violated on a Stanford site, please notify puffer-copyright-notices@cs.stanford.edu and give notice as stated under Reporting of Alleged Copyright Infringement. [2]
I'm honestly shocked this ever passed Stanford legal review. Maybe it never did?