That's how the game developers make there most money through InApp purchases take Candy Crush for example they built an empire with just that game.
I know.
call me old-school controversial stupid, but I prefer to buy game for 4.99USD (or any amount) from store, than get it "for free" and then pay for some gems or other digital goods, which makes the game somehow playable and little bit enjoyable.
The reason for InApp purchases so the game developers say is the blatant piracy of their games which maybe true but some of the games have advertisments unless you purchase items to remove them gets very tedious.