I disagree with your conclusion there. Having your clients violate the ADP TOS (and then subsequently violating it yourself while scraping data) is clearly not a smart move as illustrated by ADP's response.
Imagine Zenefits was simply a broker and your broker said to you "ADP just shut off my access to them so I can't use their service, how about we use XYZ instead?". As long as I trust my broker is providing value then it doesn't matter what payroll service I use. So, if Zenefits is confident people value their service above the service a payroll service provides, then they will most likely win in this case.