The problem is that people are misundersanding what Passbook is. It is not an app for *acquiring* cards/passes etc. It is only an electronic wallet that makes managing them easier. There are several ways to add cards, e.g. by downloading Passbook files with Safari, receive them as an attachment in an email, or through an app like United.
Another problem is that people are misunderstanding that in fact, you don't actually use Passbook per se. You're not really supposed to open it all the time and do something with it - more like Passbook is supposed to open it itself when needed and present a card or pass of some sort that is relevant to your current location and/or situation.
You use the third-party apps and websites to create Passbook passes, but also to browse locations, learn about the company, and other assorted things that you used to do before Passbook. Passbook is designed to prevent you from fumbling through the app, website, email they sent you, or screenshot you saved, when you just need the pass, gift card, or loyalty card, etc.