It's not very hard or expensive to become an "approved" developer. Only iOS devices are restricted to the App Store, Mac owners can get their apps from anywhere they choose. However, from an end-user standpoint getting apps from a "walled garden" is safer (in regards to malware and payment details being compromised) and the user is less likely to pay for totally rubbish non-usable software because of the vetting process. Yes, it has its cons, but for many non-technical users it is a fantastic solution.
Well, not really. If you're going to write an app for Apple store, first you have to be an approved developer. Then you have to have your app vetted. And correct me if I'm wrong, but Apple devices can only get apps from the Apple store.