Yes they could but why would a consumer want to do that?It's not intuitive and introduces steps where none are needed. Apple is known for making things easier not harder. How would you market "Hey if you want this app you're going to need to put down your device, go to your computer and subscribe, then you can use your app." That's the antithesis of easy.
I think the cross platform availability is smart of Apple. I'm pretty sure they realize music services need subscribers and depending on their own user base alone would not lead to a sustainable product.
My only point was that there would be a way around it if Google tried to take 30% of revenue. I don't even know if Google operates their App Store the same way Apple does. I'm guessing they don't, hence the reason so many like to complain about Apple's policies.
Yes, I'm sure that, if allowed, Apple would provide a way to subscribe directly within their Android app. But I also believe that if Google demanded 30% of revenue if it contained a direct subscription link, they would ask users to subscribe elsewhere rather than give up the revenue.