You can have and enforce a terms-of-service agreement for users without strongly and strictly identifying every user at every moment. You can enforce copyright on a dataset from a database through the courts. You do not have to allow every use just because you have allowed a certain use. These things should not be surprising.
EDIT: I googled it because I could smell the wrong in my statement, it's not called copyright it's called "database right" because a compilation of facts is not a creative work. Still the right is real regardless of the distinction.
EDIT2: CraigsList is a US company and apparently if you read deeper into the WikiPedia article, US copyrights do not respect databases and so it's not a real thing. Sorry for muddying the waters, but it doesn't change that service providers can have terms of service irrespective of how strongly they work to identify and segregate their users.
EDIT: I googled it because I could smell the wrong in my statement, it's not called copyright it's called "database right" because a compilation of facts is not a creative work. Still the right is real regardless of the distinction.
EDIT2: CraigsList is a US company and apparently if you read deeper into the WikiPedia article, US copyrights do not respect databases and so it's not a real thing. Sorry for muddying the waters, but it doesn't change that service providers can have terms of service irrespective of how strongly they work to identify and segregate their users.