BLS stands for Basic Life Support, I do not see what they can do in Cardiac Arrests. Since that is an Advanced Life Support task.
Cardiac arrest is not a "task." It's a state in which a patient is in. Chest compressions, airway maintenance, utilizing an AED, and rapid transport are all BLS "tasks" that exponentially increase a patients chance of survival. Medic's certainly have an important role, but the basics of a CPR call are extremely important and can be accomplished by a BLS unit.