It's just how Duolingo is. For things like this I tend to look up word etymology, split words up to constituent parts and compare to other words with similar parts to make sense of them. I make use of Google as Duolingo does have its limitations and this is one of them I suppose.
"It doesn't hurt to" isn't the direct translation of "Het valt te", that would be "It is worth (trying)", so that's why there's no negation in the dutch sentence. It is however used is situations where english speaker would say "It doesn't hurt to" and thus the translation.
I'm not really a dutch speaker so please correct me of I understand something wrong :)
I still find "het valt te" to be a difficult idiomatic construction to learn.
Does anybody have examples of "het valt te" used in Dutch language songs or in other memorable uses of Dutch?
By the way, "het valt te" used with other verbs might presumably not translate into "it does not hurt to".
Negation is used in translation, what word literally negate the sentence here?