As somebody who usually plays cleric, I don't mind each god being open to each domain. You're going to like the abilities from some over others, so why not let you pick the god that fits right for your character?
For NPCs I think it would make more sense to try and follow the rule, just so that as a PC you might be able to figure out what domain of cleric a character is by their god choice and vice versa, but for PCs more freedom is better
