This is a forum. There are still people who actually read what's posted to try to keep up with what is going on in the Bitcoin world. A poster who makes a habit of copy-pasting will eventually be noticed by somebody (who actually reads) and reported to the moderators.

Look at it another way: nobody will pay you to put a signature under your post if nobody is reading your post. Since advertisers are paying to put their signatures under your posts, it follows that enough people are reading your post. People who copy-paste or post shit in general will be noticed.