Ethical implications of withdrawing a paper during the rebuttal phase and submitting it somewhere else
One certainly can withdraw a paper after initial reviews and if the reviews indicate that the paper does not fit well with the conference topic-wise or has to be significantly improved/re-worked. In the latter case, the paper author should assess how much time is required to work on the paper and if it is possible to meet the deadlines (taking into account additional development, data collection/re-collection, work on the text and visualizations, literature review, etc.).
If the timeline does not seem viable, withdrawing and resubmitting to another conference is certainly the right call. In addition to the potential adaptation of the paper to the new submission, it would be very wise to use the feedback from the original reviewers to improve the paper.
I would certainly agree that in the case of withdrawal and resubmission, you would have used the reviewers' feedback and time in an intended way: to improve your work and make it better/more accessible to your reader/listener. However, there is a small catch: different entities (people, organizations) intend different things for the use of their resources, in this case, the pool of reviewers and their time.
In this case, I would say, the authors' good intentions matter the most.
If the author of the paper submitted it to a conference/journal primarily to get the feedback of the reviewers and get "free" advice knowing that the submission itself is doomed to fail — this is unethical in my opinion.
On the contrary, if a particular paper failed to go to a particular conference during peer review and its improved version ended up in another venue — this is great. Peer review and research win!
If this situation happens to the researcher consistent and too often, I would certainly agree that there is at least a hint of being a bit unethical, more from the point of being a bit lazy and wasteful of ones' peers' time.