I never understood why you can't pick up all the walls in the game. They don't do anything the walls the player already has access to can't do, and a lot of them could even be craftable (snow, ice, dirt, stone, dungeon walls from dungeon bricks). I would be alright with having it be hard-coded whether a wall type is "natural" or "artificial", so that even player-created "natural" walls would allow mobs to spawn the way naturally-generated walls in underground cabins don't, but I don't know how other players would feel about that. Maybe housing could be changed so that any space you designate as a "house" doesn't let mobs spawn, even if it's made of walls that would allow it?