There is good evidence that Apple cannot read any messages sent through iMessage with its current software. So what our politicians would want is that Apple would change their software, so that if there is evidence that a strongly suspected terrorist uses iMessage, then someone can ask Apple to give them a key that gives the government access to that terrorists iMessage account and read all his messages.
But if Apple can give some key to the government that gives the government the capability of reading one specific terrorist's iMessages, then Apple must have the capability of getting a key for any user and read that user's messages. So Apple can then read anyone's messages, which means any other government agency can issue warrants for anybody's iMessages. So no matter whether the government claims this would only affect suspected terrorists, it affects anyone.
Moreover, _someone_ at Apple would be handling these requests. That employee could be bribed, or tricked, or their computer hacked, for other governements or criminals to get access to anyone's iMessages. Including iMessages sent by military, diplomats, polticians and so on. So this thing would risk the safety of anyone. How likely is it that terrorists would find a way to exploit such a weakness to help them with their terrorist plots?