From the analysis posted, it's trying to bypass that by not sending an SMS to a premium number. It's sending the phone number to a website, and whatever it is that is running on that website is subscribing the user to a premium service.
The original idea with reverse-billing was that users could subscribe to services such as weather updates, which would automatically send a message once a day/week, and the user be charged upon receipt. The problem with reverse billing is that it's clearly open to substantial abuse.
I guess this app is going through SMS opt-in, sending the reply behind the scenes.
I wonder why the app requires SMS write permissions though.
App stores like Google Play should reward apps which require the least amount of permissions by pushing them higher in the search results (and publicize that fact).