I've never published an iOS app before. What exactly does a "developer certificate" do and why does it affect their ability to distribute internal apps in particular?
There are two certificates that are important here, with different requirements and capabilities. First, you have normal developer certificates, which you can use to submit apps to the App Store. On the other hand, you have enterprise certificates, which can be used to distribute apps outside of the App Store (and without Apple's review), and these are supposed to only be used for company use (such as internal beta testing or apps). Google and Facebook used their enterprise certificate to distribute an app to external users, most likely because the app would be rejected from the App Store, so Apple revoked their enterprise certificate.
