Is it ethical to withdraw paper that received poor reviews in first phase of reviewing and resubmit it without significant revisions?
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.