There is not clear-cut a line in English between nouns and adjectives. Nouns can generally be used attributively. Adjectives can often be used substantively.
"American" used attributively means "from America", and "American" used substantively means "(a person) from America". Semantically the difference in the case of the magazine is negligible.
Though only the founders or publishers of the magazine can say for sure, in my judgment the name simply means "(a magazine for) American persons who are scientific."