Why do some questions which have been closed as not constructive appear on the FAQ tab?
Many closed as not constructive questions are closed just because it isn't the domain of SO as Q&A site.
However they may be useful for many users, and can be asked by many users, i.e. FAQ. As it was noted in a A_funs comment
IMO it's a good thing, that high-voted questions appears in a FAQ even if they are closed as "non-constructive". Frankly speaking, when I am searching for an answer for my question, I don't care is it in domain of SO or not, I just want to find a reference to the info that many people already considered useful.