Basically every app that is used to sell goods or as an add on to a physical product doesn't have to pay Apple a cut. And it makes sense: the app is free and only the means to access a backend service or a something that was paid separately. I just don't get why it wouldn't apply to HEY: no idea of how it works, but I can imagine that for $99/year you get more than a shiny UI. That is only the means to access a backend service that is entirely independent from Apple's ecosystem (and in fact can be accessed through several channels).