The parents point is that all doors need to behave the same way. If one door can be locked, they all can be locked. The question is not "can a door be locked" but "should this door be locked so the player has to find a key?" If a player can block one door, they all need to be blockable. There should not be two doors that look identical but one will swing through a player, and the other will be blocked by the player.
It's about consistency of behavior. If one door behaves differently than another door then it needs to look different. If a door cannot be locked, then it shouldn't have a key hole. Any door with a keyhole should have a key somewhere in the level and it should be able to be locked and unlocked with that key. The only question is the starting state of the door and how many copies of the key there are.
It's not really a hard concept. It's about consistency of behavior.