This isn't technically an in-app purchase per se like a boost for your game. This is for actual commerce. Yes we have always been able to buy games and apps with Touch ID. This is expanding that to physical purchases and allows for a more secure method of purchasing between you and, lets say, Target.
Oh, I see the difference now, thanks for the explanation. When I see "in-app purchase" I tend to just think of that as "game levels" or similar. But buying physical goods is different.