For one, threat level. A cop shooting the suspect has at least the argument of supposed self-defense from imminent harm. They sought him out from a safe place with the sole purpose of his destruction.
If they had time to rig up an IED to the robot, and drive the sucker in there, use the camera to make sure he didn't have any hostages (they did that right?), and THEN detonate the bomb, it doesn't sound like the threat was all that imminent.
The FBI definition is "an individual actively engaged in killing or attempting to kill people in a confined and populated area".
He was cornered in a parking garage, not active.