Square has stated their rule is just don't do it unless it's a track specifically from Final Fantasy XIV (not even Final Fantasy at large, only this one title)

They did this because then they can say if any lawsuit ever comes up that you violated the terms of service of the game, and the issue, if any person who owns rights to the song, can go after you specifically as opposed to SquareEnix. They're not going to give us a broader, more involved or complex list of "this is also okay if..." because they don't want any ambiguity or deniability on the part of the player that can pass the responsibility back onto them as a company.

Not only that, though, but it's possible even then that Square could still be held liable as they're collecting a fee that could be argued to be making money off the protected work of others. They want to make it absolutely clear that they want zero responsibility for anyone violating any usage rights out there, and there's no benefit for them to say that one thing is okay because it then can become ambiguous unless they make a list of literally every single song that is okay from what is currently public domain and what is still held in copyright by a company or composer's estate. They don't want to be a part of all that legal stuff, nor do they want to have to have every person in the game understand it all. It's better for them to keep the rules simple.