You got it pretty much on point. Shooting a laser at atoms is like shooting a machine gun at an indestructible target. If it moves towards you, you can slow it down. But preventing it from accelerating when the target is stationary is where quantum mechanics comes in. That is your explanation: The laser light only acts as a force when the light is resonant with the atom and the Doppler effect means that the resonance condition changes depending on the speed of the atoms.