Zenefits' entire service is about re-selling and managing other services. That's like saying an accountant is at fault for relying on payroll processors that may someday decide they want to be accountants too - of course they're not at fault, that's how accounting is DONE. This is ridiculous on ADPs part.
I'm not close enough on the details so I could be wrong. But it sounds like a) Zenefits integrates to ADP and starts being the flow through for payroll. b) Zenefits decides to become a direct competitor to ADP by launching their own payroll service (ZenPayroll mentioned here) c) ADP isn't enthusiastic about supporting a service that plans to upsell or cross sell shared customers to move away from them.
No offense, but this is standard business practice. Integration hooks are added to an application PURELY as a feature to make the application more compelling. Uses of the integration hooks which are not making the application more compelling will probably be undermined eventually.
Ultimately Zenefits is in this situation because they are more or less a UI or Orchestrator of other services that do the 'real' work. ADP was just tired of letting a middle man eat off of them.
