When they advertise through other channels, a significant fraction of the users they get will turn out to be iOS users, who, regardless of what the CTA is, will

When they advertise through other channels, a significant fraction of the users they get will turn out to be iOS users, who, regardless of what the CTA is, will respond by installing it on their phone through the Apple Store. They start out as free users. Apple then disallows any attempt to convert them into paid users via the app, and also via email, and also via pretty much any targeted channel. So Spotify would have to not only give up on the channel, but also mostly give up on that customer segment.