There are many reasons you might want to outlaw or ban something even when there isn’t an easy way to detect it. Case in point: the existence of this law, even in the absence of enforceability, makes “using AI for a song intended for the charts” into an active and intentional act of deception by the artist. That is a moral threshold that will deter the most marginal of the bad actors, and supplement the willpower of the insufficently principled good actors.