The Crimson Binome
Hero
Fluff and crunch are both equally mandatory. You can change the fluff if you don't like it. You can change the crunch if you don't like it. Either one can have room for interpretation. It's your game.So basically you are saying that "fluff is RAW" or FAW ("fluff as written") is mandatory.
It's going to vary from setting to setting, as much of it is left vague. The PHB doesn't tell us much about them, so we have to fill in the blanks based on what is there. My interpretation is that it's either the local nature deity (Ceres or Idunn or what-have-you), or an even older pantheon which has fallen out of common worship.And who exactly are the "powers that be" granting powers to druids according to the PHB?
It's kind of like in AD&D, where there wasn't a default setting so much, and the only thing we knew was that priests weren't supposed to use swords (except for druids, who could use scimitars but weren't supposed to wear metal armor). There's still a reason why they can't do those things, but we're left to our own interpretations as to why exactly.