Square Enix does.
A question to you: who decides if I am allowed to express my opinion, you?
While people can change their name IRL, their previous name still exists in the database and can be seen by the authorities in case something comes up, so people can't just run away by changing their names. This is different in this case.People can change names in real life as well as in many other games. What is your rationale for making name changes invalid in FF14?
edit: "different" being that the authorities (SE) is less interested in that information than the player community, and the player community won't have access to that info.