The concern is that, say, a musician is in the process of making a new song. Google sees this, and releases the song shortly before the artist releases it thems

The concern is that, say, a musician is in the process of making a new song. Google sees this, and releases the song shortly before the artist releases it themselves, and for free! Now what is the artist to do? The likelihood? Small. But if it happens, you'll have Google befitting from that artist's work, and the artist less able to do so. The artist would have given away his right to sue, possibly unknowingly (because who reads TOS'?!) And of course Google won't pay him. Not because they don't want to, but because they know it'll open the floodgates of similar claims!