This is a way to bootstrap an app economy and solve the chicken and egg problem. If they launch with their own app store (like Nokia) no one will build for it and they won't get any device sales because they have no apps for the device. This way they get 200k+ apps ready to go and if they can build a better piece of hardware, they'll get a big install base. Then they can convince developers to develop native apps for their platform.
What incentive is there to create great Playbook Native apps when you can just make 1 web app that "will work" in a "browser" for Playbook?
Developers who really care about the platform will create native apps, and ones who merely want Playbook users to be able to run their app will go with some cross platform thing. (The web, Android, etc).