What can I do, if I don't like the proposed research subject by my potential supervisor? I am supposed to be admitted to a M.A.Sc.  graduate program in engineering at a reputable Canadian university. I am now communicating with my potential supervisor about the potential plans for my research...  and here is my concern:

My background and enthusiasm have been shaped around applications research and implementation of the acquired control techniques within the realm of robotics, whereas my supervisor has a more theoretical viewpoint. When we had started to communicate, he did assert that we would determine the research theme,  according to our common interests and overlapping subjects.  But today, I have received an email from him,  consisting of an idea for research that is considerably theoretical. 

I am not really attracted by that subject, since it does not cover my desired research field, considerably.  I might notify my idea,  frankly,  but I am afraid of confronting him and creating any dissatisfaction from him. 
He is eager to hear my viewpoint,  and I have no idea how to transfer my perspective,  efficiently,  such that any misunderstanding will be avoided. 

Have I right in this case,  or have to do whatever my potential supervisor is saying? 

I have not yet met him,  face to face,  but I know that he is,  amazingly,  kind and able to track what's going in my mind.  How should I declare my opinion about the concern,  such that he would not consider my clarification as a disobeying?
A successful project profits from both supervisor and candidate being interested in the same question. If you really dislike the topic, you should try to convince the supervisor to accept a different question. He will then, if he is amenable, try to convince you of a variation of his, you will amend it, etc. until - hopefully - convergence.

However, you may find that the supervisor is simply not interested in your direction. This can happen, even if he likes you. That's fine, and then you either have to find somebody closer to your question of interest or compromise.

That being said, it seems that you are talking about an MA Sc degree. This is considerably shorter than a PhD, so, depending on the details of the situation, it may be easier for you to find a compromise.

If topic really matters for you, however, you need to think hard whether you want to go into a field which you have qualms in. 

I am not sure about what the ethical consideration of a "theoretical topic" as opposed to a practical one could possibly be; be is as it may, I would avoid bringing this up, as it may insinuate that you cast doubts about the ethics of your prospective supervisor.