Ghostbusters? Are FS going down the Zen route now?
You know there are mods to real pinball tables too right?
Anyway I think it's just a test of the marketplace for things like that outside of TPA.
While I agree that it would make more sense to bundle it into Pinball Arcade, it would also be harder to promote it that way.
The reason to have it as its own app is for publicity, so it can have its own entry on the store.
Even Zen do that, they have all tables available under the main app but also standalone versions of the different licensed bundles, so that they get more publicity when launched than plain DLC does.