Hypothetical scenario. Bob joins the group and during our session 0 tells a quick story about how he's integrating into the campaign and that he grew up loving nature.
DM: cool, that works for everyone, what class are you thinking?
Bob: Druid.
DM: Awesome, that's a good addition to the group. By the way, I shouldn't have to say this but I follow the rules except for my house rules on my campaign intro page. Druids will not wear metal armor.
Bob: Too bad. My druid is going to buck the system and wear metal armor as soon as he can find some or afford it.
DM: No, druids will not wear metal armor.
Bob: Mine will. Freedom!!!
DM: You could play a nature cleric...
Bob: Nope. The rules don't say that the druid can't wear metal armor and you aren't the boss of me.
DM: The rules clearly state that they will not. I agree that I am not the boss of you, but the PHB is clear. I can go out to your car and pour a gallon of water into it's gas tank but I will not. I'm willing to work with you here, it's a fantasy game and alternative materials can be used. Non-metallic scale male is a thing, it's just slightly more expensive.
Bob. Armor made of shiny, shiny metal is all that will do.
DM: Well, sorry this isn't going to work out I'll have to let someone on the waiting list know they can join instead.