Terms of service usually constitute a legal agreement between the site owner and the viewer, which most often can be held up in a court of law. Different sites use different terms that suit their business. Pornographic sites use the terms required to legally view explicit material, and to ward off underage children attempting to view the material that might cause a parent to take legal action against them. Supremacist sites also use an agreement that frees them of any legal trouble that may arise from hateful content, or content being passed through their servers via a third party. Beastiality sites generally refer to terms similar to pornographic sites, but are generally more strict about the area the viewer is from.
It's important to be stringent on your own site's T.O.S. if you're providing questionable content that could present some form of a legal issue. I've read several states are looking to force website owners to record the real names, addresses, and other information of their viewers and keep them
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