That isn't necessary and may just lead to an accidental fuel starvation thus giving you a whole new set of problems to work through.
Not to mention lateral control problems in the air. The aircraft starts letting you know there is an imbalance even with 2~3 tonnes difference between opposite tanks.
One thing I was mildly surprised about was how long after landing the engines appear (from the head-on footage) to have been kept running after coming to a stop.
Perhaps the absence of fire indications on the flight deck? Perhaps the flight crew thought along the lines that if they switched off the engines, they've committed themselves to a full scale evacuation in the immediate future, with: Cabin emergency light batteries running flat, loss of airconditioning (even on cold days, a full aircraft gets warm pretty quickly with all the doors closed), power to passenger address systems depleting, etc. With an evacuation by slides, you run the risk of injury to passengers.
The tower told them to look for cues from the fire service. In this case, does the fire service contact them on VHF or do they use some kind of hand signals?