They're not real Final Fantasys, but they're not "cheap knock offs," either. They're actually parts of completely different series that were saddled with the Final Fantasy name in America because, at the time, Square didn't think its games would sell without the name recognition value of FF.

Final Fantasy Adventure is actually Seiken Densetsu, the first installment in what has become a series of four games (plus one remake). Final Fantasy Legend I-III are actually SaGa 1-3, the opening trio in another series that has spanned nine games so far.