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Yep. In most cases, it's part of the TOS. I can't recall ever seeing one that didn't
The "Why" is really quite simple...It give the provider the ability to control content posted and moderate as they wish.
Not only forums, but if you were to peruse any service that you use for communication (phone, cable TV, etc) you'll most likely see something in the fine print saying that they(or law enforcement) agencies can do whatever they want,
I know a fellow who tried to do that. All the people who had ideas went away to find another forum.
Thank you for your reply GJ and Happy Sunday! :)

They don't actually claim ownership. The TOS says you retain ownership, but the site gets a free license to use or sell your material without any compensation to you. Same thing only different.