"He is a danger to society."
Translation:C'est un danger pour la société.
In this specific case, "he is a danger..." may not translate in "c'est un danger..." if you want to make clear that you are talking about a human being, for lack of context.
However, the rule proposed by Duolingo remains valid in all cases where "he is" or "she is" or "they are" are followed by a determinant (article, possessive adjective) and a noun: c'est un..., ce sont des...
If you get the French sentence first, you can translate: it is a danger, she is a danger or he is a danger.
"C'est un danger" is still the only accepted answer. More people reporting this should draw dou's attention to the issue.
i'm also very puzzeled in that it is not accepted. "C'est.." for "he is" to me sounds outright wrong.
I would suggest not to spend much time on questions of this type (c'est vs il est) until you reach an advanced level in French, it can be extremely confusing for beginners or intermediates in my opinion.
According to the website french.com (http://french.about.com/od/grammar/a/preposition_pour.htm) pour can be used to express a point of view
Pour nous, c'est une bonne idée. - For us, it's a good idea.
Il est tout pour moi.- He is everything to me.
C'est un mystère pour moi. - It is a mystery to me.
Read the comments. C'est more indicates an event here than a person. It is unclear.