I wouldn't post this if I didn't think that this is going to create a bad example for future decisions. We are drawing lines about what is appropriate too strictly. We are being too conservative, the default position should be trusting decisions made by other users. If there is a disagreement, if the OP comes back and says I want this removed for some reasons, then we can discuss it, but as long as there is not a conflict, we should be very liberal about what users can do.
What are we going to do if a user copies a deleted answer and posts it? Are we going to down vote it? Are we going to delete it? What if a number of users undelete a post. The system permits other users to undelete posts, even if it is deleted by the OP. Are we going to punish the users for undeleting it? Are we going to delete it again?
I also don't see any reason legally or ethically for not copying an answer. What is the ethical principle that is being violated?
IMHO, copying answers is completely compatible with the philosophy of the site. Let me quote a part of FAQ:
Other people can edit my stuff?!
Like Wikipedia, this site is collaboratively edited, and all edits are tracked. If you are not comfortable with the idea of your questions and answers being edited by other trusted users, this may not be the site for you.
I think this is clear about the philosophy of SE. If you are not happy about copying answers, I respect your view. But if you are saying that it is inappropriate or unethical and/or if you are trying to draw lines too strictly about how other users or moderators should act, and about what is appropriate and what is not, sorry, but I don't agree.
I agree with Tsuyoshi that there is not a need to agree on everything, i.e. I agree with Tsuyoshi to disagree.
My main point here is on how conservative we should be on what trusted users/moderators can do. My view, as I wrote above, is that we should trust decisions made by others and be very liberal, we don't need to discuss every decision made by others, unless there is a conflict/disagreement.
Let me also make it clear that there is a difference between being nice to others and what is ethical. An action can be considered "not being nice to others", but still don't violate any ethical principles. Being nice is boarder than being ethical (in this context).
Now, let me return to the particular case of this question.
My sentence about the ethical principle being violated is in the context of our discussion, i.e. the appropriate use of this site. The argument, if I understood it correctly, is that we should respect the decision made by OP about deleting their post, and I guess this is based on the view that OP own what they have posted. I am not sure I agree with this. Again if I understood the philosophy of the site correctly, it is very similar to Wikipedia, and it is a collaborative effort. If you have posted something, others can modify, edit, or even copy-and-post it.
Some discussion on why it is not clear to me if a post is owned by OP, specially a CW post:
I. When someone posts an answer/questions, it means that other users are probably going to avoid posting similar content,
II. People vote for the answer,
III. Other answers may refer to it.
Removing a post can also be considered as not respecting the questioner, other user answering the question, and the votes given to the answer.
Now, if it is CW, i.e. community wiki, it is more involved.
IV. CW answers are owned by community, they are edited by other users, and removing a post can cause even more problems in this case.
Assume that we have a CW question about what is the most famous problem in TCS, and someone posts "$P$ vs $NP$", and of course receive lots of votes, after a few months the OP goes back and removes the answer. Assume that no one notices this. Think how it would look like if someone from outside or a new user reads the question and does not see an "$P$ vs $NP$" answer?! And even if another users posts it again, how many times do other uses have to vote for the same answer? Should we expect them to vote once every time someone removes it and someone else reposts an answer which is essentially the same?
I suggest you read this post: What can cause a post to be deleted, and what does that actually mean?
In particular, this part:
When can't I delete my own post?
You can't delete your answer if it has been accepted.
You can't delete your question if it has more than two answers, if an answer has a score >= 2, or it has been closed less than 48 hours ago.
There are restrictions on when an answer can be removed by OP.
I. it is not clear to me that OP owns what they post, specially CW posts,
II. deleting a good highly-voted post by OP (without stating any reasons) can also be considered disrespect towards other users, and may cause problems.
Just to make it clear, I agree that we should be nice to others, and if the OP complains about undeleting their post (and states some reasons for it) I would support the OP, but IMO, the default position should be that "posts can be undeleted by others, even if OP have removed it", specially if they are good and highly-voted.