I think the reasoning would also be fairly similar to the reason that the big players are unbundling their apps.
The most valuable real estate is the homescreen of your phone. A website bookmark isn't likely to get that placement. A website bookmark that you can download from the App Store on the other hand...
I think it's easy, but not quite obvious to the non-technical user. Then they have the issue that once they've tapped on the icon on the homescreen, they are thrown into Safari. And doing the multi-tasking gesture shows them Safari, not the website directly as a first class app, so it's not obvious how to get back. If they tap again and the URL is not identical to what they are currently viewing, they will get a new tab. So there's good reasons to ship an app, even it is just a webview.
The most valuable real estate is the homescreen of your phone. A website bookmark isn't likely to get that placement. A website bookmark that you can download from the App Store on the other hand...