Yoshi=P is saying it's semantics. All the guild's are named by their class, but FFXIV can't have an active Thieve's Guild openly operating. It goes against all current lore and would be bad roleplaying (I know. "What's that?" , but stay with me.). Think about it. How many open thieves do you meet in real life? However you might meet several people who have job that employ a rogue skillset including military, police, magicians, etc. Plain and simple, thief has a negative connotation that no person of good legal standing wants associated with them. The only reason they got away with it in XI is that no real plot weight is given to any standard class in that game.