"sugar" is a noun while "sweet" is an adjective. While you may feel that the sentences mean the same, they differ grammatically. Duo is concerned with making you aware of the grammatical structure; after all, for example, you do need to be able to differentiate between nouns and adjectives when deciding whether it needs to be capitalized in German.
DL is a word to word teaching system, it rather focuses on the grammar skills and vocab and tries to help understand the rules of the language, than giving beautiful literary sentences to translate. For me this system is working better, so I can accept and forgive for silly sentences like this. And at the mean time I do not think that ever German would use this expression in real life. If you started this course from the very beginning than at this point you can probably understand that with the approximately 40 nouns that we know officially, sometimes we just don't have better examples to practice on. :)
Has anyone ever heard anyone say "The orange(s) have/has sugar"? No. This is a very strange sentence. The oranges contain sugar. I assumed that, like Hunger, this wasn't truly a "noun" being "had" but was a kind of adjective. "Ich habe Hunger" is "I am hungry", ergo "Die Orangen haben Zucker." must mean "Oranges are sweet".
But no, it had to be a really awkward sentence which no person has ever said.