Also, from the video, I notice they only kick him in the head until they get the knife off him so they were only disarming an armed attacker.
I noticed that as well. After being told multiple times to drop the knife and being kicked in the head, he still held on to the knife. Maybe it is just me, but once you are on the ground with police on top of you kicking you in the head, it may be time to simply drop the knife.
The inability or unwillingness to follow simple instructions appears to be a recurring theme among the criminal element both here and abroad.