I just went digging around the zenefits.com and pulled this verbiage out, which i think outlines the fundamental business problem: 'Zenefits works with all top payroll providers, so there's no need to switch from your favorite system.' ADP doesn't want to be a 'provider' in the 'Payroll As A Service' sense. Further Zenefits entry point into the market is based on the low friction of you not needing to leave your curr…
1) If you're choosing a payroll provider, Zenefits integration may help you choose ADP because you have a non-awful user experience with Zenefits as a front-end.
2) It could run afoul of antitrust law for tying, monopolizing, or the essential facilities doctrine (http://en.wikipedia.org/wiki/Aspen_Skiing_Co._v._Aspen_Highl...), if the case could be made that ADP has sufficient market power.