It is legal and very common. It is done simply to limit the liability in unforeseen events. Some courts can throw this point out during the case, some may adhere to that. But most terms on the Internet would include it "just in case".
And no, we won't duplicate customers' works and such — we have build it for photographers, and we are photographers ourselves. We care about that stuff.
True. But we want you to have the truthful information in case someone steals _your_ photos. We'll be able to investigate and find out the truth. Having real name, web-site, twitter or facebook helps us a lot in that.
There has been a few cases like that, and photographers love that we take proactive approach in solving their problems.
We don't check, but our community self-monitors and reacts very quickly (few minutes at most).