In a game, if the GM agrees the rules they are using is "druids will not wear metal armor" then that GM wont be mind controlling PC druids to fo do or forcing them into traps where "the only choice" is to do do. That would be violating the agreement, not unlike giving paladins a no-win alignment test in ye olde days.
In a game, if the player agrees the rules they are using is "druids will not wear metal armor" then that player wont opt for donning metal armor as an easier or quicker way around a challenge and will find other ways, other options.
So, in a game where both the players and the gm play by the rules they agree to, the difference between "will not" and "cannot" is nada.
For a forum sock puppet rager tho...
The point isn't even a matter of "cannot" or "will not", which I'd have hoped is obviously clear by now, but I guess it's not that easy for those with lesser reading comprehension. Even if it said "cannot", it would still be just as busted of a rule, because it still wouldn't fit into the game system. It fits as well and is just as enforceable as when being a Dwarf granted proficiency in throwing hammers (note: that item doesn't exist). Unless something imposes a mechanical reason as to why an action is impossible for a character to perform, the player can choose to have that character perform that action. That's how it works for Paladins, that's how it works for Clerics, that's how it works for Warlocks, and that's how it works for Druids. That's simply how the game works. Period.