If ADP doesn't want to work with a competing third party that is their right. This is why you should never base a business on the goodwill of a platform. We saw the same shenanigans all the time in social and mobile gaming.
Zenefits isn't relying on goodwill -- they're actually a subcontractor of the customer of ADP, so ADP is actually blocking their own (paying) customers. ADP is saying that they'll delete the logins of customers who use a particular subcontracting firm, because they don't want their customers to use that firm's services.
I think you don't quite understand what the situation is here.
