"The app stores, like GooglePlay, iTunes and Amazon each sell a different version of apps exclusive to the device platform. They are in competition with one another, and do not share their sales or profits.

That means a copy purchased from GooglePlay cannot be downloaded again for free to a Apple device using iTunes. A new copy will need to be purchased separately for the Apple device. Same if the situation is reversed.

This also is their policy for all IAPs (In App Purchases) made from each store."