As far as I understand Zenefits is doing both screen scraping and manual entry into the ADP systems and they may have never even talked to ADP. So instead of hitting an API directly where ADP may expect a large amount of requests Zenefits is hitting their normal website requesting hundreds or thousands of users every hour.

I would have blocked them too. It looks far too much like malice from the ADP side of things where it could be bot nets or something else versus a legit company.