Cancelling all existing subscriptions seems extremely hostile. Imagine if you were a happy customer and suddenly Google rips the service away from you. This looks really bad for Google. I’m sure it was a mistake to handle it this way and I’m sure they will fix this one off but it’s scary to know this can happen. Also I sure hope they restart the subscriptions. What a sticky situation they’ve caused. Imagine you are subscribed. Then you’re subscription is cancelled. Then it’s started again. We’re they notifying the customers? That’s gotta cause some churn.
Devil's advocate: it's probably not due to the "download apple music" button and more to do with the tagline that came with it: "Millions of songs. All ad-free. New users can get upto 3 months free.".
>or is this just normal external service webview based login?
is the tagline part of the webview, or is it content controlled by you, and the webview only opens when you tap the button? if it's the former then I'd agree it's not an ad, although I can still understand why it might look like an ad to a reviewer who doesn't understand the nuances.