To avoid the button that prompts for purchase, put the device in airplane mode. Start the game, you get a notification that the game can't connect to the iTunes store ( thats a good thing) and then will get prompted for your name, and so on. After all that, you can put the device back in regular connectivity, and you won't see the prompt to purchase.
This seems to be common to all Playrix games when they are free.
OTOH, once the free period is over, and you have the full version, you also won't be prompted for purchase. It's just how they do their games. They are all similar, but also all pretty fun if you like match 3s.