Correct me if I'm wrong, but I think (personally) the purpose of these kind of sentences or any sentence on duolingo is to give tou some building blocks in a language, and you're suppose to take it apart and make different sentences with them. Plus I'm just starting Russian, so I need some bare basic vocab to get me started. I dunno thats just what I think about it
Yes. "Это цирк, не кафе" is right too. But without the "а" there should be too big pause between "цирк" and "не". Huge pauses make some people uncomfortable, because it sounds like there is a dot: "Это цирк. Не кафе." And with that sounding-like-dot-pause the centence sounds pretty strange. So usually we use "1, а не 2" instead of "1, не 2".