Shouldn't this be written "Un bon homme"? Aren't goodness/badness qualifiers the one of the exceptions that come before the noun in French?
The B.A.G.S. rule is based on subjective/ figurative adjectives going before the noun. Most adjectives that fall into the B.A.G.S. category are pretty subjective so using B.A.G.S. is a shortcut for placement rather than dwelling on subjectivity versus objectivity.
If you refer to a man as being good because that is your opinion, that is subjective/ figurative so the adjective goes in front. If you refer to a man as being good because he has won awards for being good, that is objective/ literal so the adjective goes after the noun.
By placing the adjective where he has the author is saying, you can call a man good even if doesn't respect animals but he won't be certified as good unless he does.
Actually Duo is just trying to get you to notice that sometimes good goes in front and sometimes after.
For more than you probably want to know about this subject see:
I guess this one is an exception. "un bonhomme" (a guy) also exists, so to avoid confusion, I assume the noun and adjective have been switched.
for other adjective placement rules : http://french.about.com/od/grammar/a/adjectives_4.htm
From what I understand, this is to emphasize that you are talking about a good man, as opposed to men who are not. By moving the adjective from its usual place, you are setting apart "a good man" from those who are not good.