The text says to include the essential parts of the answer. If the author of the answer does that and only that, it would fall under the fair use doctrine - see the U.S. Copyright Office's information on Fair Use. But if the author, as you say, copies and pastes a large portion of copyrighted text, presenting it as their own, that would be both legally and morally wrong.
When you discover plagiarism or copyright violation, you should flag it to the moderators and let them decide how to take actions. See e.g. "Possible plagiarism detected?" and "Post deleted due to DMCA notice" on meta.SO.