The smartphone is probably the most sensitive device most people own. It knows your location always. It has your banking apps. Your password manager. Your instant messages, and social media chats, it knows whether you’re walking, or driving, or talking on the phone, and to whom.
Once Apple allows any other vendor to vacuum all of that intensively private information out of an iPhone, Apple becomes indirectly responsible for potentially massive privacy breaches.
Legally, maybe not, practically it becomes their problem.