A phone and a laptop are two different things, and it makes sense to lock down one in a way that it doesn't make sense to lock down the other. There is no indication that Apple doesn't understand that.

What I see with App Store sandboxing is a way to allow people to more easily try and buy apps without worrying what those apps are doing to their computer. I'm very careful about downloading and trying apps on my Mac because I don't know what files they're going to leave around on my computer or worse. Sandboxing in the App Store is a great way to make that situation better.