What is the strategy behind this? Obviously they don't care if they got caught. So is it some form of demonstrating who has got the power in the country?

I truly don't know. The officially stated reason doesn't check for the motives. Not all the cars in the K-Mart car park had potentially offensive objects in them. It may still be orders from above, it looks too systematic to be just a spur of the moment to let off steam during a stressful deployment by getting back at hypothetical car-owning protesters.

Maybe they really are afraid someone is going to use their car against them, so they take preventative measures by disabling any car in their immediate vicinity so that it doesn't happen.

Either way you slice it it's bad. Scorched earth policies can only "work" when deployed in a foreign country. But maybe that's what these out-of-towner police officers feel like with their military camouflage attire.

Possibly it's really this military feeling. AFAIK the equipment you use, does have a feedback on yourself.

But this is more an explanation for the end of the command chain, not for the head.

The leadership has a punitive mentality. They believe if the people are punished, then they will fall in line like dogs.
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Die Führung haben eine strafende Mentalität. Sie glauben, wenn die Leute bestraft werden, werden sie wie Hunde gehorchen.