It would seem logical in that downloading the app it shows intent to purchase eventually and the structure of iOS allows for less distractions than browsing via the web.

If I'm using an iPad app, it occupies my entire screen and has my full attention (minus alerts that may arrive) and therefore I'm less likely to pulled away by something else (twitter stream, messages from friends, links, etc.).

Also, if I'm launching a shopping app, it's also done with intent to shop.