The point of a copyright notice, is exactly to let other people know what they are allowed to do with the information that is being released. It's in some respects like a party invitation - being invited to someone's house on a certain date and time, does NOT give you permission to come into that person's home without notice at any other time. Even if I use the traditional phrase of departure to you, and say "my home is always open to you," that does not mean I want you to come uninvited into my house and keep my wife company when I am gone. Doing that in many places around the world is a license to getting yourself killed without a question.
Likewise, posting something in a public place is not only NOT an invitation to use it as if it were yours, but is traditionally, especially in the US and other UK-influenced countries, the means to make sure that others CANNOT use it as if it were freely available. A copyright notice is used to let other people know that it is okay to do certain things with the revealed information. A warning is just a courtesy to remind someone that it is not okay to treat that something as if it had not author/owner/protector.
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The point of a copyright notice, is exactly to let other people know what they are allowed to do with the information that is being released. It's in some respects like a party invitation - being invited to someone's house on a certain date and time, does NOT give you permission to come into that person's home without notice at any other time. Even if I use the traditional phrase of departure to you, and say "my home is always open to you," that does not mean I want you to come uninvited into my house and keep my wife company when I am gone. Doing that in many places around the world is a license to getting yourself killed without a question. Likewise, posting something in a public place is not only NOT an invitation to use it as if it were yours, but is traditionally, especially in the US and other UK-influenced countries, the means to make sure that others CANNOT use it as if it were freely available. A copyright notice is used to let other people know that it is okay to do certain things with the revealed information. A warning is just a courtesy to remind someone that it is not okay to treat that something as if it had not author/owner/protector.