If a government investigator joins a WhatsApp channel where loads of people are sharing CSE, WhatsApp will help the government find the people responsible. WhatsApp encrypts the content of the data, but they retain message logs and do cooperate with law enforcement. Presumably the same for iMessage.
This largely conforms with how the first telecoms received immunity for abuse of their services. They retain logs and assist the government with investigations, and in exchange they are shielded from liability. WhatsApp and iMessage would probably cooperate to the same extent, minus wire-tapping messages in transit (because they can't). That's vastly greater cooperation than a tor exit node operator that retains no logs.s
This largely conforms with how the first telecoms received immunity for abuse of their services. They retain logs and assist the government with investigations, and in exchange they are shielded from liability. WhatsApp and iMessage would probably cooperate to the same extent, minus wire-tapping messages in transit (because they can't). That's vastly greater cooperation than a tor exit node operator that retains no logs.s