In the case of Hey they don't want to have ONLY App Store subscriptions, they want to support people who don't use iPhones. That means they'll have to set up a

In the case of Hey they don't want to have ONLY App Store subscriptions, they want to support people who don't use iPhones. That means they'll have to set up a whole extra infrastructure for dealing with app store subscriptions on top of the infrastructure for dealing with their own subscriptions. That's a non-trivial amount of work and ongoing support they didn't want to do.