Earlier quoted context omitted.
I don't think shooting him would have been legal either. You only get to shoot a suspect when there's an imminent threat, and it sounds like there wasn't one. (If there was, they wouldn't have had time to send in the robot.)
That's an excellent point, that in order to bring in a robot they more or less pre-meditated executing him in the field and likely means the suspect had not moved to escalate the situation. You don't prep the bomb and then only set it off once the perp has made his choice to hurt someone else.
You can't just "wait him out" and hope he won't escape.