I think it's because it's linked your Square Enix account. If you notice, you need to log onto the Square Enix Account System in order to get on the forums. So, if your "forum" account is compromised, then your game account/lodestone/whatever else is on there is compromised too.

It's a tossup between a secure account vs a convenient one. It would be a little annoying if "unaffiliated parties" joined the forums... like spam bots or trolls with throwaway accounts.

A little silly, yes, but in the long run it's more beneficial.