I don't know about other states, but Colorado has these stupid "Posted" signs everywhere. For some reason, people think the sign they post needs to say "Posted". The very act of posting the sign makes it posted, it doesn't need to say so. That's like printing "Book" in giant letters on the cover of every book, just in case people don't know what it is.
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IMO it's a way of calling attention to the sign but since the isn't enough room for ATTENTION they instead put POSTED. I could be totally off base too.