They demonstrate compliance, impact resistance and the dynamic nature of their control algorithms. It will not break, and it'll keep it's balance. Compliance is also a safety feature when the robot is working around humans. Most of the mechanical design, including gearboxes, must be designed to resist outside impacts, because these impacts cause much larger torques on the joints than the motors itself.

It's much easier to make an 'animatronic' robot which has pre-programmed motions and can't withstand mechanical impacts.

Demonstrating a use case for their customers (likely law enforcement).

To demonstrate robustness against disturbances. I'm not sure if it has to be abusive, though.

Indeed, this is a pet peeve of mine as well. If the silly notion of Roko’s Basilisk has any merit, I wouldn’t want to be that guy. Besides, transversing an unstable or uneven surface would seem to be a better test, anyway.

I do however find it amusing that it is also apparently a requirement for a humanoid robot to stand up in the most uncanny way possible.