I am not a lawyer, but to copy my comment below: They ask for donations to cover their expenses for hosting the archive, and to defend creators on the basis of fair use in a transformative work (not exactly the terms of art, but close). They don't charge a fee to use the website either to post fan works or to look at them. They make no profit, they don't vet creators, they don't edit the works, they don't advertise the works...they perform none of the functions of publisher.