True, but the police can do a lot of things to people that civilians are not allowed to do to each other - including, for instance, pulling a driver over for speeding and asking for I.D., completely innocuous, non-physical things like that.
You call the police when a situation has escalated beyond your means to control it, and they step in with more serious responses than a civilian is trained to deal with. They had a paramedic on the scene, for one thing.
Of the means available to them, pepper spray probably beats putting the kid into a headlock, or going after him in full battle gear (needed to protect their own faces from the kid's sharpened stick), using tear gas, or other stronger responses.