The goal in this tree is to learn to speak Romanian naturally, not natural English. Natural Romanian is "fruits." If I start translating words like "fructe" as singular I'm likely to forget it's plural in Romanian.
"We give you fruits" is not the most natural way to say it, but it is not worse English then "we give you fruit." Fruits is not commonly used, but it appears in well-known phrases ("fruits of their labor"), a google reveals people using it to talk about multiple kinds of fruit, etc.
The wording of English sentences has nothing to do with how natural the Romanian is. The Romanian AND English sentences should sound natural, otherwise they're just propagating bad language.
And let's not forget that sometimes people take Duolingo course in languages other than their native one. For them, poor English sentences are misleading.
I don't understand why you're insisting "we give you fruits" is bad language. Fruits is a perfectly valid form of the word in English. It's less common then fruit, but uncommon != bad. There's actually a good wiktionary write-up of the issue: https://en.wiktionary.org/wiki/fruit#Usage_notes
Teaching most of your Romanian language students bad Romanian because some of them would learn an uncommon word in English ("fruits" as a noun) does not make sense to me.
@nickbii fruit is overwhelmingly used as an uncountable noun, especially in the context that this lesson is implying. Your link supports this as well.
I'm not saying that the sentence with "fruits" shouldn't be accepted as a correct answer but the default answer given should align with standard usage unless there is a compelling reason not to. Certainly, valuing literalness in a default translation over naturalness is not a compelling reason.
It appears to me that "fructe" means both "fruit" and "fruits"; then there is the word "fructa: (with the "a" having the accent breve diacritic mark); how is it pluralized, I wonder. [By the way, I also note that the acute accent (') is used in some words to indicate a stressed vowel.]