I don't think they're talking about posting your work for free on the internet, they mean when you submit an assignment for a class turnitin stores your assignm

I don't think they're talking about posting your work for free on the internet, they mean when you submit an assignment for a class turnitin stores your assignment so they can check future submissions against yours. In this case, you haven't (necessarily) shared it anywhere public, all you did was submit it to your teacher, and now turnitin gets a copy forever.