An app, a beer, a song, a burger. The list could go on forever.
Some of the things we get are ephemeral, others are permanent—or at least less ephemeral. If you have to pay $2,99 for a song, you would probably think about it. But paying $2,99 for a beer when your are out with your friends is reasonable.
The beer is a once-in-a-lifetime experience, while the song will be yours forever. The same happens with apps—where once you buy a product you can get future uploads for free. What has more value?