The difference I think is the scale and name recognition of Shazam and wanting to get new customers via Apple Music. They can even say something like "with an iPhone you don't even need an app for Shazam, you can just use Siri", but that works better if non-Apple users know what Shazam is. while this they will probably retire to be used as the nameless backend to the default weather app.

Still, killing DarkSky on Android seems to offer little gain while hurting DarkSky supporters and generating ill will. I hope Apple and DarkSky can reconsider this decision.