Is it ethical to withdraw paper that received poor reviews in first phase of reviewing and resubmit it without significant revisions? Many conferences in my field use two-phase reviewing: in the first phase, 2-3 reviewers review the paper; their reviews are made available to the authors; the authors are given a chance to respond; and then in the second phase, the reviewers read the authors' response and the program chairs might optionally solicit additional reviews.

If a paper receives poor reviews after the first phase, so that it looks like the paper is likely to be rejected, is it ethical to withdraw the paper and submit it elsewhere, without making significant changes?  

I read Ethical implications of withdrawing a paper during the rebuttal phase and submitting it somewhere else, which asks about a similar question, but I am interested in the specific case where the authors were well-intended (they submitted to the first conference with the legitimate hope it would be accepted) but do not plan to make significant revisions before re-submitting -- a case that is not covered by the answers there.  For instance, it is not uncommon to submit to a selective conference in hopes that it will be accepted, discover that the reviewers don't consider it strong enough for publication there, and then consider submitting to a less selective conference.  (So it's not that the paper is flawed, but it isn't strong enough in its current form.)
There is absolutely no ethical concern or constraint at all here. The reviews you get in academia for such works are a gift to the community and to you. You have complete control over the paper until you give up your rights to it. The reviewers aren't paid by the conferences (or journals). If I as a reviewer help you improve a paper you have submitted to conference A and it later appears at conference B instead, everyone has gained. And you aren't bound to take my advice. There is no contract to that effect. If I review your paper, I get no rights to take over its content or ideas. 

Moreover, it may be that the paper without change is actually appropriate elsewhere and the reviews were colored to some extent by the nature of the conference. 

Most academic conferences aren't held as profit making concerns and nearly everyone involved is a volunteer (some support staff may be paid). It is a service we do th the idea of scholarship itself, nothing more. 

If I give you a gift, it is yours to use as you like or not and welcome. You have taken nothing that wasn't given freely. 

Over the course of your life you have gotten a lot of advice. Some of it you even solicited. But much of it you probably ignored. It was advice, nothing more. Use it if it is valuable. 



While there are no ethical concerns, there are some practical ones. The sets of reviewers in the various CS conferences overlap somewhat. And if you got particular reviewer for one, there is a non-zero chance the same person will come up again, especially for topics with few available reviewers.