I wonder if these fights are just disinformation to try to convince criminals/terrorists that they can use iMessage. The government lets a criminal get away with it in a case they don't really care about or can convict them without it anyways and makes a lot of press, and then has access to it in all the cases they do care about.
iMessage is designed with warrants in mind if you read over the protocol documentation. Each device has its own key and is tied to your Apple Id. If you have a iPhone, a Macbook, and an iPad each device has its own encryption key. When someone sends you an iMessage, Apples sends them the public key for each of the 3 devices and then the encrypted message is sent to each device which uses its private key to decrypt the message.
When a warrant is issued, all Apple has to do is add a 4th, "FBI device" to your Apple Id and anyone sending you an iMessage also gets encrypted with that key.
As Apple controls the user interface and they provide no way to view how many keys an iMessage is being encrypted with, there is no easy way to see if an extra key for ease-dropping is being used. There may be ways if one monitored the size of the traffic, but I am not aware of that work being done. Anyone who had the need to make sure they weren't being spied on by the government, wouldn't use iMessage.