Kirk McElhearn, writing for his blog:

When Apple updated the Music app in iOS 8.4, the company added Apple Music. This streaming service is tightly integrated into iOS (and iTunes, on the desktop). In fact, it’s so well integrated that you can’t turn it off.

There’s a setting in Settings > Music that lets you toggle Apple Music’s visibility. But there’s nothing that turns Apple Music off entirely. You can see this by asking Siri to play some music. As long as you have an Apple Music subscription – trial or paid – and you’re signed into your account, Siri will play music from Apple Music, sometimes even when you already have music by the same artist on your iOS device.

I wonder if that’s why the Settings switch is labeled Show Apple Music instead of Enable Apple Music or some such. I also wonder if that behavior will continue with iOS 9.