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.
Zenefits wasn't a competitor to ADP, but it seems that ADP thinks that they may want to roll out a product that competes with Zenefits, so now they are trying to clear the playing field before they step onto it.
This would be similar to Apple deciding to screw up some 3rd party apps that run on OSX because they are working on a product that competes (but has not been announced or released yet). (And then you coming along claiming that said 3rd party software was a 'competitor' to Apple, so should just 'suck it up').
I like how you specified "on OSX" because this is exactly what Apple has done multiple times on iOS. Remember Maps?
This model was well-established long before Zenefits hit the scene.
By your account, Walmart should manufacture all the items it sells, etc.