It amazes me how some of you are so confused by this.
The in app purchase gimmick is rampent and sick. The app maker make a watered down product and then ask you to add on to it, at a price, to make it the way it should have been at time of download.
FREE ain't always FREE.
Angry Birds is one game in particular that does this sh**. You notice that as you progress through AB the physics changes.HMMM. At 45 degrees I should get max distance all the time , huh. Yeah right....