I still consider Metroid Prime to be an adventure, with first person shooting elements. I mainly view this because when the term "FPS" springs to mind, you think about multiple players killing eachother in an arena. Everyone knows that Metroid Prime is a very different game to say Halo or Timesplitters (games you would define as an fps). Besides in MP, you don't actually aim (even though you can) you just lock on and shoot, pretty much unheard of in FPSs.
But I agree with you about Zelda. Zelda is clearly not an RPG.