And if he never called them, how are they part of any "Operation" he might have been involved in?
Easily - in fact the very fact that call data would be available to law enforcement is reason enough to discourage contacting them over the phone unless it was an emergency (and probably in the middle of an 'operation'). You can also use various sorts of social media to connect with co-conspirators, which are not subject to the same kinds of logs as phone calls, and the data are generally encrypted so although you might already know that he used a particular platform, you don't necessarily know who he contacted or what information was exchanged. You can also use apps like Skype, which doesn't go through the normal phone system.
Don't get me wrong, I don't think that they're likely to find much more than they already have even if they manage to decrypt the phone, although they might get lucky if the terrorist had taken better-than-average precautions - but they're desperately looking anything to make sure that they've neutralized as many of the threats as possible. That's certainly a legitimate concern, but sometimes there's no good answer, and this is one of them; installing a backdoor creates a virtual certainty that it will be misused for less benign purposes.
It is not reasonable to say that that they should shoot to disable rather than to kill the shooter when that might mean that more people might lose their lives. It's actually much harder to shoot to disable rather than to shoot to kill, because merely hitting somebody in a limb won't disable them sufficiently, and shooting them in their core body is fairly likely to be fatal (even if not immediately) - yet another situation where there are no good answers.