There is a difference between a walled garden for the sake of capturing users, and a walled garden because it makes sense.
Having a universal app store would be a pain the neck because of all the different platforms that you would have to support. It makes a lot of sense to have one app store per platform. If I'm on an iPhone, why would I want my interface to be cluttered up by Android apps or vice versa.
Nor does an app store mean that the host has to decide who gets in and who doesn't. Some platforms do, some don't. Even something like Debian that runs an app repository (which is what an app store is, sans payment system) has guidelines for submission - the key that differentiates it from the app store is that you can run unapproved code.
So a system where each platform has its own app store and apps that don't go through it are allowed to run on the platform make the most sense. Platforms that do not allow installing thigns outside their model are deficient, but this is a not a knock on app-stores in general.
An app store gives somewhere for vendors to hang up their shingle (like a mall), and -- depending on the barriers to entry -- gives some assurance to "shoppers" looking for apps.
We've just launched a new business app, bSelling - http://bcisive.austhink.com/solutions/bselling, on Salesforce.com's App Exchange for add-ons to Salesforce CRM, and wish that there were more such stores.
It's actually a bit like the Microsoft stranglehold on the old PC software market; there were certainly negative aspects, but the standardization effect was certainly market-making.
Having a universal app store would be a pain the neck because of all the different platforms that you would have to support. It makes a lot of sense to have one app store per platform. If I'm on an iPhone, why would I want my interface to be cluttered up by Android apps or vice versa.
Nor does an app store mean that the host has to decide who gets in and who doesn't. Some platforms do, some don't. Even something like Debian that runs an app repository (which is what an app store is, sans payment system) has guidelines for submission - the key that differentiates it from the app store is that you can run unapproved code.
So a system where each platform has its own app store and apps that don't go through it are allowed to run on the platform make the most sense. Platforms that do not allow installing thigns outside their model are deficient, but this is a not a knock on app-stores in general.