You know what the best way is to see who cooperates with law enforcement and to what level? Court documents!
I've been trawling court documents for the past few months (I'm writing a blog article on this) and I'm yet to find iMessage being used in court (unless the access to the conversation was given by one of the parties). iMessage really does seem secure from legal system point of view.
For a comparison, I've found dozens of court documents from Google, Facebook, Microsoft handing over chat logs from Hangouts, Whatsapp/FBMessenger, Skype.
Anyway, if you want to see the level of cooperation and want to double-check privacy policies, actual court documents are the best way.