Are you aware of the conditions of any purchase at the app store. You might want to read that; a purchase cannot be transferred and if a new version is introduced, the old version becomes unusable and you need to purchase the new one.
If you don't like the conditions attached to app store purchases, don't buy anything there or you agree to the terms.
http://www.apple.com/legal/internet-services/itunes/ww/
Pick your country.
Here is one of the applicable paragraphs (this is the US version):
As an accommodation to you, subsequent to acquiring Eligible Content, you may download certain of such previously-purchased Eligible Content onto any Associated Device. Some Eligible Content that you previously purchased may not be available for subsequent download at any given time, and Apple shall have no liability to you in such event. As you may not be able to subsequently download certain previously-purchased Eligible Content, once you download an item of Eligible Content, it is your responsibility not to lose, destroy, or damage it, and you may want to back it up.