A lot of the Siri IP belonged to Stanford SRI and Siri was basically licensing it. So when Adam left Siri and started Viv, Apple couldn't do anything. Also note

A lot of the Siri IP belonged to Stanford SRI and Siri was basically licensing it. So when Adam left Siri and started Viv, Apple couldn't do anything. Also note that, Viv rarely talked about any details of their technology. Probably they were slightly afraid. At the same time Apple didn't want to pick a fight with a small startup and damage it's reputation. It might change now that Samsung is the owner.